diff --git a/libraries/stdlib/js/src/kotlin/text/StringBuilderJs.kt b/libraries/stdlib/js/src/kotlin/text/StringBuilderJs.kt
index 91c6559804b..88d74f2f956 100644
--- a/libraries/stdlib/js/src/kotlin/text/StringBuilderJs.kt
+++ b/libraries/stdlib/js/src/kotlin/text/StringBuilderJs.kt
@@ -46,8 +46,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
         return this
     }
 
-    @OptIn(ExperimentalStdlibApi::class)
-    actual override fun append(value: CharSequence?, startIndex: Int, endIndex: Int): StringBuilder = this.appendRange(value, startIndex, endIndex)
+    actual override fun append(value: CharSequence?, startIndex: Int, endIndex: Int): StringBuilder =
+        this.appendRange(value ?: "null", startIndex, endIndex)
 
     /**
      * Reverses the contents of this string builder and returns this instance.
@@ -96,7 +96,6 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      * and then that string was appended to this string builder.
      */
     @SinceKotlin("1.3")
-//    @ExperimentalStdlibApi
     actual fun append(value: Boolean): StringBuilder {
         string += value
         return this
@@ -107,9 +106,10 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * Characters are appended in order, starting at the index 0.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun append(value: CharArray): StringBuilder {
+        @OptIn(ExperimentalStdlibApi::class)
         string += value.concatToString()
         return this
     }
@@ -118,7 +118,6 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      * Appends the specified string [value] to this string builder and returns this instance.
      */
     @SinceKotlin("1.3")
-//    @ExperimentalStdlibApi
     actual fun append(value: String): StringBuilder {
         this.string += value
         return this
@@ -131,8 +130,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * In Kotlin/JS implementation of StringBuilder the value returned from this method may not indicate the actual size of the backing storage.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun capacity(): Int = if (this.asDynamic()._capacity !== undefined) maxOf(this.asDynamic()._capacity, length) else length
 
     /**
@@ -144,8 +143,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      * In Kotlin/JS implementation of StringBuilder the size of the backing storage is not extended to comply the given [minimumCapacity],
      * thus calling this method has no effect on the further performance of operations.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun ensureCapacity(minimumCapacity: Int) {
         if (minimumCapacity > capacity()) {
             this.asDynamic()._capacity = minimumCapacity
@@ -157,8 +156,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * Returns `-1` if the specified [string] does not occur in this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun indexOf(string: String): Int = this.string.asDynamic().indexOf(string)
 
     /**
@@ -167,8 +166,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * Returns `-1` if the specified [string] does not occur in this string builder starting at the specified [startIndex].
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun indexOf(string: String, startIndex: Int): Int = this.string.asDynamic().indexOf(string, startIndex)
 
     /**
@@ -177,8 +176,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * Returns `-1` if the specified [string] does not occur in this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun lastIndexOf(string: String): Int = this.string.asDynamic().lastIndexOf(string)
 
     /**
@@ -187,8 +186,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * Returns `-1` if the specified [string] does not occur in this string builder starting at the specified [startIndex].
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun lastIndexOf(string: String, startIndex: Int): Int {
         if (string.isEmpty() && startIndex < 0) return -1
         return this.string.asDynamic().lastIndexOf(string, startIndex)
@@ -202,8 +201,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun insert(index: Int, value: Boolean): StringBuilder {
         AbstractList.checkPositionIndex(index, length)
 
@@ -216,8 +215,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun insert(index: Int, value: Char): StringBuilder {
         AbstractList.checkPositionIndex(index, length)
 
@@ -232,11 +231,12 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun insert(index: Int, value: CharArray): StringBuilder {
         AbstractList.checkPositionIndex(index, length)
 
+        @OptIn(ExperimentalStdlibApi::class)
         string = string.substring(0, index) + value.concatToString() + string.substring(index)
         return this
     }
@@ -251,8 +251,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun insert(index: Int, value: CharSequence?): StringBuilder {
         AbstractList.checkPositionIndex(index, length)
 
@@ -268,8 +268,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun insert(index: Int, value: Any?): StringBuilder {
         AbstractList.checkPositionIndex(index, length)
 
@@ -282,8 +282,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun insert(index: Int, value: String): StringBuilder {
         AbstractList.checkPositionIndex(index, length)
 
@@ -302,8 +302,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      *  @throws IndexOutOfBoundsException or [IllegalArgumentException] if [newLength] is less than zero.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun setLength(newLength: Int) {
         if (newLength < 0) {
             throw IllegalArgumentException("Negative new length: $newLength.")
@@ -323,8 +323,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * @throws IndexOutOfBoundsException if [startIndex] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun substring(startIndex: Int): String {
         AbstractList.checkPositionIndex(startIndex, length)
 
@@ -336,8 +336,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of this string builder indices or when `startIndex > endIndex`.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun substring(startIndex: Int, endIndex: Int): String {
         AbstractList.checkBoundsIndexes(startIndex, endIndex, length)
 
@@ -353,8 +353,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * In Kotlin/JS implementation of StringBuilder the size of the backing storage is always equal to the length of the string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     actual fun trimToSize() {
         if (this.asDynamic()._capacity !== undefined) {
             this.asDynamic()._capacity = length
@@ -379,8 +379,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * @throws IndexOutOfBoundsException if [index] is out of bounds of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     public operator fun set(index: Int, value: Char) {
         AbstractList.checkElementIndex(index, length)
 
@@ -396,8 +396,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * @throws IndexOutOfBoundsException or [IllegalArgumentException] if [startIndex] is less than zero, greater than the length of this string builder, or `startIndex > endIndex`.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     public fun setRange(startIndex: Int, endIndex: Int, value: String): StringBuilder {
         checkReplaceRange(startIndex, endIndex, length)
 
@@ -423,8 +423,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * @throws IndexOutOfBoundsException if [index] is out of bounds of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     public fun deleteAt(index: Int): StringBuilder {
         AbstractList.checkElementIndex(index, length)
 
@@ -440,8 +440,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] is out of range of this string builder indices or when `startIndex > endIndex`.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     public fun deleteRange(startIndex: Int, endIndex: Int): StringBuilder {
         checkReplaceRange(startIndex, endIndex, length)
 
@@ -461,8 +461,8 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      * @throws IndexOutOfBoundsException when the subrange doesn't fit into the [destination] array starting at the specified [destinationOffset],
      *  or when that index is out of the [destination] array indices range.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     public fun toCharArray(destination: CharArray, destinationOffset: Int = 0, startIndex: Int = 0, endIndex: Int = this.length) {
         AbstractList.checkBoundsIndexes(startIndex, endIndex, length)
         AbstractList.checkBoundsIndexes(destinationOffset, destinationOffset + endIndex - startIndex, destination.size)
@@ -484,9 +484,10 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      *
      * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] array indices or when `startIndex > endIndex`.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     public fun appendRange(value: CharArray, startIndex: Int, endIndex: Int): StringBuilder {
+        @OptIn(ExperimentalStdlibApi::class)
         string += value.concatToString(startIndex, endIndex)
         return this
     }
@@ -494,16 +495,15 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
     /**
      * Appends a subsequence of the specified character sequence [value] to this string builder and returns this instance.
      *
-     * @param value the character sequence from which a subsequence is appended. If [value] is `null`,
-     *  then characters are appended as if [value] contained the four characters `"null"`.
+     * @param value the character sequence from which a subsequence is appended.
      * @param startIndex the beginning (inclusive) of the subsequence to append.
      * @param endIndex the end (exclusive) of the subsequence to append.
      *
      * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] character sequence indices or when `startIndex > endIndex`.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
-    public fun appendRange(value: CharSequence?, startIndex: Int, endIndex: Int): StringBuilder {
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
+    public fun appendRange(value: CharSequence, startIndex: Int, endIndex: Int): StringBuilder {
         val stringCsq = value.toString()
         AbstractList.checkBoundsIndexes(startIndex, endIndex, stringCsq.length)
 
@@ -524,11 +524,12 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] array indices or when `startIndex > endIndex`.
      * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     public fun insertRange(index: Int, value: CharArray, startIndex: Int, endIndex: Int): StringBuilder {
         AbstractList.checkPositionIndex(index, this.length)
 
+        @OptIn(ExperimentalStdlibApi::class)
         string = string.substring(0, index) + value.concatToString(startIndex, endIndex) + string.substring(index)
         return this
     }
@@ -539,17 +540,16 @@ public actual class StringBuilder actual constructor(content: String) : Appendab
      * The inserted characters go in the same order as in the [value] character sequence, starting at [index].
      *
      * @param index the position in this string builder to insert at.
-     * @param value the character sequence from which a subsequence is inserted. If [value] is `null`,
-     *  then characters will be inserted as if [value] contained the four characters `"null"`.
+     * @param value the character sequence from which a subsequence is inserted.
      * @param startIndex the beginning (inclusive) of the subsequence to insert.
      * @param endIndex the end (exclusive) of the subsequence to insert.
      *
      * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] character sequence indices or when `startIndex > endIndex`.
      * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
-    public fun insertRange(index: Int, value: CharSequence?, startIndex: Int, endIndex: Int): StringBuilder {
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
+    public fun insertRange(index: Int, value: CharSequence, startIndex: Int, endIndex: Int): StringBuilder {
         AbstractList.checkPositionIndex(index, length)
 
         val stringCsq = value.toString()
@@ -575,8 +575,8 @@ public actual inline fun StringBuilder.clear(): StringBuilder = this.clear()
  *
  * @throws IndexOutOfBoundsException if [index] is out of bounds of this string builder.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @Suppress("EXTENSION_SHADOWED_BY_MEMBER", "NOTHING_TO_INLINE")
 public actual inline operator fun StringBuilder.set(index: Int, value: Char) = this.set(index, value)
 
@@ -589,8 +589,8 @@ public actual inline operator fun StringBuilder.set(index: Int, value: Char) = t
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] if [startIndex] is less than zero, greater than the length of this string builder, or `startIndex > endIndex`.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @Suppress("EXTENSION_SHADOWED_BY_MEMBER", "NOTHING_TO_INLINE")
 public actual inline fun StringBuilder.setRange(startIndex: Int, endIndex: Int, value: String): StringBuilder =
     this.setRange(startIndex, endIndex, value)
@@ -604,8 +604,8 @@ public actual inline fun StringBuilder.setRange(startIndex: Int, endIndex: Int,
  *
  * @throws IndexOutOfBoundsException if [index] is out of bounds of this string builder.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @Suppress("EXTENSION_SHADOWED_BY_MEMBER", "NOTHING_TO_INLINE")
 public actual inline fun StringBuilder.deleteAt(index: Int): StringBuilder = this.deleteAt(index)
 
@@ -617,8 +617,8 @@ public actual inline fun StringBuilder.deleteAt(index: Int): StringBuilder = thi
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] is out of range of this string builder indices or when `startIndex > endIndex`.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @Suppress("EXTENSION_SHADOWED_BY_MEMBER", "NOTHING_TO_INLINE")
 public actual inline fun StringBuilder.deleteRange(startIndex: Int, endIndex: Int): StringBuilder = this.deleteRange(startIndex, endIndex)
 
@@ -634,8 +634,8 @@ public actual inline fun StringBuilder.deleteRange(startIndex: Int, endIndex: In
  * @throws IndexOutOfBoundsException when the subrange doesn't fit into the [destination] array starting at the specified [destinationOffset],
  *  or when that index is out of the [destination] array indices range.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @Suppress("EXTENSION_SHADOWED_BY_MEMBER", "NOTHING_TO_INLINE", "ACTUAL_FUNCTION_WITH_DEFAULT_ARGUMENTS")
 public actual inline fun StringBuilder.toCharArray(destination: CharArray, destinationOffset: Int = 0, startIndex: Int = 0, endIndex: Int = this.length) =
     this.toCharArray(destination, destinationOffset, startIndex, endIndex)
@@ -651,8 +651,8 @@ public actual inline fun StringBuilder.toCharArray(destination: CharArray, desti
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] array indices or when `startIndex > endIndex`.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @Suppress("EXTENSION_SHADOWED_BY_MEMBER", "NOTHING_TO_INLINE")
 public actual inline fun StringBuilder.appendRange(value: CharArray, startIndex: Int, endIndex: Int): StringBuilder =
     this.appendRange(value, startIndex, endIndex)
@@ -660,17 +660,16 @@ public actual inline fun StringBuilder.appendRange(value: CharArray, startIndex:
 /**
  * Appends a subsequence of the specified character sequence [value] to this string builder and returns this instance.
  *
- * @param value the character sequence from which a subsequence is appended. If [value] is `null`,
- *  then characters are appended as if [value] contained the four characters `"null"`.
+ * @param value the character sequence from which a subsequence is appended.
  * @param startIndex the beginning (inclusive) of the subsequence to append.
  * @param endIndex the end (exclusive) of the subsequence to append.
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] character sequence indices or when `startIndex > endIndex`.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @Suppress("EXTENSION_SHADOWED_BY_MEMBER", "NOTHING_TO_INLINE")
-public actual inline fun StringBuilder.appendRange(value: CharSequence?, startIndex: Int, endIndex: Int): StringBuilder =
+public actual inline fun StringBuilder.appendRange(value: CharSequence, startIndex: Int, endIndex: Int): StringBuilder =
     this.appendRange(value, startIndex, endIndex)
 
 /**
@@ -686,8 +685,8 @@ public actual inline fun StringBuilder.appendRange(value: CharSequence?, startIn
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] array indices or when `startIndex > endIndex`.
  * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @Suppress("EXTENSION_SHADOWED_BY_MEMBER", "NOTHING_TO_INLINE")
 public actual inline fun StringBuilder.insertRange(index: Int, value: CharArray, startIndex: Int, endIndex: Int): StringBuilder =
     this.insertRange(index, value, startIndex, endIndex)
@@ -698,16 +697,15 @@ public actual inline fun StringBuilder.insertRange(index: Int, value: CharArray,
  * The inserted characters go in the same order as in the [value] character sequence, starting at [index].
  *
  * @param index the position in this string builder to insert at.
- * @param value the character sequence from which a subsequence is inserted. If [value] is `null`,
- *  then characters will be inserted as if [value] contained the four characters `"null"`.
+ * @param value the character sequence from which a subsequence is inserted.
  * @param startIndex the beginning (inclusive) of the subsequence to insert.
  * @param endIndex the end (exclusive) of the subsequence to insert.
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] character sequence indices or when `startIndex > endIndex`.
  * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @Suppress("EXTENSION_SHADOWED_BY_MEMBER", "NOTHING_TO_INLINE")
-public actual inline fun StringBuilder.insertRange(index: Int, value: CharSequence?, startIndex: Int, endIndex: Int): StringBuilder =
+public actual inline fun StringBuilder.insertRange(index: Int, value: CharSequence, startIndex: Int, endIndex: Int): StringBuilder =
     this.insertRange(index, value, startIndex, endIndex)
diff --git a/libraries/stdlib/jvm/src/kotlin/text/StringBuilderJVM.kt b/libraries/stdlib/jvm/src/kotlin/text/StringBuilderJVM.kt
index bc6602f4d0f..25668bc7be6 100644
--- a/libraries/stdlib/jvm/src/kotlin/text/StringBuilderJVM.kt
+++ b/libraries/stdlib/jvm/src/kotlin/text/StringBuilderJVM.kt
@@ -33,8 +33,8 @@ public actual inline operator fun StringBuilder.set(index: Int, value: Char): Un
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] if [startIndex] is less than zero, greater than the length of this string builder, or `startIndex > endIndex`.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @kotlin.internal.InlineOnly
 public actual inline fun StringBuilder.setRange(startIndex: Int, endIndex: Int, value: String): StringBuilder =
     this.replace(startIndex, endIndex, value)
@@ -48,8 +48,8 @@ public actual inline fun StringBuilder.setRange(startIndex: Int, endIndex: Int,
  *
  * @throws IndexOutOfBoundsException if [index] is out of bounds of this string builder.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @kotlin.internal.InlineOnly
 public actual inline fun StringBuilder.deleteAt(index: Int): StringBuilder = this.deleteCharAt(index)
 
@@ -61,8 +61,8 @@ public actual inline fun StringBuilder.deleteAt(index: Int): StringBuilder = thi
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] is out of range of this string builder indices or when `startIndex > endIndex`.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @kotlin.internal.InlineOnly
 public actual inline fun StringBuilder.deleteRange(startIndex: Int, endIndex: Int): StringBuilder = this.delete(startIndex, endIndex)
 
@@ -78,8 +78,8 @@ public actual inline fun StringBuilder.deleteRange(startIndex: Int, endIndex: In
  * @throws IndexOutOfBoundsException when the subrange doesn't fit into the [destination] array starting at the specified [destinationOffset],
  *  or when that index is out of the [destination] array indices range.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @kotlin.internal.InlineOnly
 @Suppress("ACTUAL_FUNCTION_WITH_DEFAULT_ARGUMENTS")
 public actual inline fun StringBuilder.toCharArray(destination: CharArray, destinationOffset: Int = 0, startIndex: Int = 0, endIndex: Int = this.length) =
@@ -96,8 +96,8 @@ public actual inline fun StringBuilder.toCharArray(destination: CharArray, desti
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] array indices or when `startIndex > endIndex`.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @kotlin.internal.InlineOnly
 public actual inline fun StringBuilder.appendRange(value: CharArray, startIndex: Int, endIndex: Int): StringBuilder =
     this.append(value, startIndex, endIndex - startIndex)
@@ -105,17 +105,16 @@ public actual inline fun StringBuilder.appendRange(value: CharArray, startIndex:
 /**
  * Appends a subsequence of the specified character sequence [value] to this string builder and returns this instance.
  *
- * @param value the character sequence from which a subsequence is appended. If [value] is `null`,
- *  then characters are appended as if [value] contained the four characters `"null"`.
+ * @param value the character sequence from which a subsequence is appended.
  * @param startIndex the beginning (inclusive) of the subsequence to append.
  * @param endIndex the end (exclusive) of the subsequence to append.
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] character sequence indices or when `startIndex > endIndex`.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @kotlin.internal.InlineOnly
-public actual inline fun StringBuilder.appendRange(value: CharSequence?, startIndex: Int, endIndex: Int): StringBuilder =
+public actual inline fun StringBuilder.appendRange(value: CharSequence, startIndex: Int, endIndex: Int): StringBuilder =
     this.append(value, startIndex, endIndex)
 
 /**
@@ -131,8 +130,8 @@ public actual inline fun StringBuilder.appendRange(value: CharSequence?, startIn
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] array indices or when `startIndex > endIndex`.
  * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @kotlin.internal.InlineOnly
 public actual inline fun StringBuilder.insertRange(index: Int, value: CharArray, startIndex: Int, endIndex: Int): StringBuilder =
     this.insert(index, value, startIndex, endIndex - startIndex)
@@ -143,18 +142,17 @@ public actual inline fun StringBuilder.insertRange(index: Int, value: CharArray,
  * The inserted characters go in the same order as in the [value] character sequence, starting at [index].
  *
  * @param index the position in this string builder to insert at.
- * @param value the character sequence from which a subsequence is inserted. If [value] is `null`,
- *  then characters will be inserted as if [value] contained the four characters `"null"`.
+ * @param value the character sequence from which a subsequence is inserted.
  * @param startIndex the beginning (inclusive) of the subsequence to insert.
  * @param endIndex the end (exclusive) of the subsequence to insert.
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] character sequence indices or when `startIndex > endIndex`.
  * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 @kotlin.internal.InlineOnly
-public actual inline fun StringBuilder.insertRange(index: Int, value: CharSequence?, startIndex: Int, endIndex: Int): StringBuilder =
+public actual inline fun StringBuilder.insertRange(index: Int, value: CharSequence, startIndex: Int, endIndex: Int): StringBuilder =
     this.insert(index, value, startIndex, endIndex)
 
 
diff --git a/libraries/stdlib/src/kotlin/text/Appendable.kt b/libraries/stdlib/src/kotlin/text/Appendable.kt
index cabb8699a78..8a396d2bb65 100644
--- a/libraries/stdlib/src/kotlin/text/Appendable.kt
+++ b/libraries/stdlib/src/kotlin/text/Appendable.kt
@@ -42,16 +42,15 @@ expect interface Appendable {
 /**
  * Appends a subsequence of the specified character sequence [value] to this Appendable and returns this instance.
  *
- * @param value the character sequence from which a subsequence is appended. If [value] is `null`,
- *  then characters are appended as if [value] contained the four characters `"null"`.
+ * @param value the character sequence from which a subsequence is appended.
  * @param startIndex the beginning (inclusive) of the subsequence to append.
  * @param endIndex the end (exclusive) of the subsequence to append.
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] character sequence indices or when `startIndex > endIndex`.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
-public fun <T : Appendable> T.appendRange(value: CharSequence?, startIndex: Int, endIndex: Int): T {
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
+public fun <T : Appendable> T.appendRange(value: CharSequence, startIndex: Int, endIndex: Int): T {
     @Suppress("UNCHECKED_CAST")
     return append(value, startIndex, endIndex) as T
 }
diff --git a/libraries/stdlib/src/kotlin/text/StringBuilder.kt b/libraries/stdlib/src/kotlin/text/StringBuilder.kt
index 9c16f87f87a..f9388f9dc16 100644
--- a/libraries/stdlib/src/kotlin/text/StringBuilder.kt
+++ b/libraries/stdlib/src/kotlin/text/StringBuilder.kt
@@ -64,7 +64,6 @@ expect class StringBuilder : Appendable, CharSequence {
      * and then that string was appended to this string builder.
      */
     @SinceKotlin("1.3")
-//    @ExperimentalStdlibApi
     fun append(value: Boolean): StringBuilder
 
     /**
@@ -72,15 +71,14 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      * Characters are appended in order, starting at the index 0.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun append(value: CharArray): StringBuilder
 
     /**
      * Appends the specified string [value] to this string builder and returns this instance.
      */
     @SinceKotlin("1.3")
-//    @ExperimentalStdlibApi
     fun append(value: String): StringBuilder
 
     /**
@@ -88,8 +86,8 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      * The capacity is the maximum length this string builder can have before an allocation occurs.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun capacity(): Int
 
     /**
@@ -98,8 +96,8 @@ expect class StringBuilder : Appendable, CharSequence {
      * If the current capacity is less than the [minimumCapacity], a new backing storage is allocated with greater capacity.
      * Otherwise, this method takes no action and simply returns.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun ensureCapacity(minimumCapacity: Int)
 
     /**
@@ -107,8 +105,8 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      * Returns `-1` if the specified [string] does not occur in this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun indexOf(string: String): Int
 
     /**
@@ -117,8 +115,8 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      * Returns `-1` if the specified [string] does not occur in this string builder starting at the specified [startIndex].
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun indexOf(string: String, startIndex: Int): Int
 
     /**
@@ -127,8 +125,8 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      * Returns `-1` if the specified [string] does not occur in this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun lastIndexOf(string: String): Int
 
     /**
@@ -137,8 +135,8 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      * Returns `-1` if the specified [string] does not occur in this string builder starting at the specified [startIndex].
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun lastIndexOf(string: String, startIndex: Int): Int
 
     /**
@@ -149,8 +147,8 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun insert(index: Int, value: Boolean): StringBuilder
 
     /**
@@ -158,8 +156,8 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun insert(index: Int, value: Char): StringBuilder
 
     /**
@@ -169,8 +167,8 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun insert(index: Int, value: CharArray): StringBuilder
 
     /**
@@ -183,8 +181,8 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun insert(index: Int, value: CharSequence?): StringBuilder
 
     /**
@@ -195,8 +193,8 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun insert(index: Int, value: Any?): StringBuilder
 
     /**
@@ -204,8 +202,8 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun insert(index: Int, value: String): StringBuilder
 
     /**
@@ -219,8 +217,8 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      *  @throws IndexOutOfBoundsException or [IllegalArgumentException] if [newLength] is less than zero.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun setLength(newLength: Int)
 
     /**
@@ -228,8 +226,8 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      * @throws IndexOutOfBoundsException if [startIndex] is less than zero or greater than the length of this string builder.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun substring(startIndex: Int): String
 
     /**
@@ -237,8 +235,8 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of this string builder indices or when `startIndex > endIndex`.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun substring(startIndex: Int, endIndex: Int): String
 
     /**
@@ -248,8 +246,8 @@ expect class StringBuilder : Appendable, CharSequence {
      * then it may be resized to become more space efficient.
      * Calling this method may, but is not required to, affect the value of the [capacity] property.
      */
-    @SinceKotlin("1.3")
-    @ExperimentalStdlibApi
+    @SinceKotlin("1.4")
+    @WasExperimental(ExperimentalStdlibApi::class)
     fun trimToSize()
 }
 
@@ -267,8 +265,8 @@ public expect fun StringBuilder.clear(): StringBuilder
  *
  * @throws IndexOutOfBoundsException if [index] is out of bounds of this string builder.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 public expect operator fun StringBuilder.set(index: Int, value: Char)
 
 /**
@@ -280,8 +278,8 @@ public expect operator fun StringBuilder.set(index: Int, value: Char)
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] if [startIndex] is less than zero, greater than the length of this string builder, or `startIndex > endIndex`.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 public expect fun StringBuilder.setRange(startIndex: Int, endIndex: Int, value: String): StringBuilder
 
 /**
@@ -293,8 +291,8 @@ public expect fun StringBuilder.setRange(startIndex: Int, endIndex: Int, value:
  *
  * @throws IndexOutOfBoundsException if [index] is out of bounds of this string builder.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 public expect fun StringBuilder.deleteAt(index: Int): StringBuilder
 
 /**
@@ -305,8 +303,8 @@ public expect fun StringBuilder.deleteAt(index: Int): StringBuilder
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] is out of range of this string builder indices or when `startIndex > endIndex`.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 public expect fun StringBuilder.deleteRange(startIndex: Int, endIndex: Int): StringBuilder
 
 /**
@@ -321,8 +319,8 @@ public expect fun StringBuilder.deleteRange(startIndex: Int, endIndex: Int): Str
  * @throws IndexOutOfBoundsException when the subrange doesn't fit into the [destination] array starting at the specified [destinationOffset],
  *  or when that index is out of the [destination] array indices range.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 public expect fun StringBuilder.toCharArray(destination: CharArray, destinationOffset: Int = 0, startIndex: Int = 0, endIndex: Int = this.length)
 
 /**
@@ -336,23 +334,22 @@ public expect fun StringBuilder.toCharArray(destination: CharArray, destinationO
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] array indices or when `startIndex > endIndex`.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 public expect fun StringBuilder.appendRange(value: CharArray, startIndex: Int, endIndex: Int): StringBuilder
 
 /**
  * Appends a subsequence of the specified character sequence [value] to this string builder and returns this instance.
  *
- * @param value the character sequence from which a subsequence is appended. If [value] is `null`,
- *  then characters are appended as if [value] contained the four characters `"null"`.
+ * @param value the character sequence from which a subsequence is appended.
  * @param startIndex the beginning (inclusive) of the subsequence to append.
  * @param endIndex the end (exclusive) of the subsequence to append.
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] character sequence indices or when `startIndex > endIndex`.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
-public expect fun StringBuilder.appendRange(value: CharSequence?, startIndex: Int, endIndex: Int): StringBuilder
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
+public expect fun StringBuilder.appendRange(value: CharSequence, startIndex: Int, endIndex: Int): StringBuilder
 
 /**
  * Inserts characters in a subarray of the specified character array [value] into this string builder at the specified [index] and returns this instance.
@@ -367,8 +364,8 @@ public expect fun StringBuilder.appendRange(value: CharSequence?, startIndex: In
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] array indices or when `startIndex > endIndex`.
  * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
 public expect fun StringBuilder.insertRange(index: Int, value: CharArray, startIndex: Int, endIndex: Int): StringBuilder
 
 /**
@@ -377,17 +374,16 @@ public expect fun StringBuilder.insertRange(index: Int, value: CharArray, startI
  * The inserted characters go in the same order as in the [value] character sequence, starting at [index].
  *
  * @param index the position in this string builder to insert at.
- * @param value the character sequence from which a subsequence is inserted. If [value] is `null`,
- *  then characters will be inserted as if [value] contained the four characters `"null"`.
+ * @param value the character sequence from which a subsequence is inserted.
  * @param startIndex the beginning (inclusive) of the subsequence to insert.
  * @param endIndex the end (exclusive) of the subsequence to insert.
  *
  * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [startIndex] or [endIndex] is out of range of the [value] character sequence indices or when `startIndex > endIndex`.
  * @throws IndexOutOfBoundsException if [index] is less than zero or greater than the length of this string builder.
  */
-@SinceKotlin("1.3")
-@ExperimentalStdlibApi
-public expect fun StringBuilder.insertRange(index: Int, value: CharSequence?, startIndex: Int, endIndex: Int): StringBuilder
+@SinceKotlin("1.4")
+@WasExperimental(ExperimentalStdlibApi::class)
+public expect fun StringBuilder.insertRange(index: Int, value: CharSequence, startIndex: Int, endIndex: Int): StringBuilder
 
 @Suppress("EXTENSION_SHADOWED_BY_MEMBER")
 @Deprecated("Use append(value: Any?) instead", ReplaceWith("append(value = obj)"), DeprecationLevel.WARNING)
@@ -450,7 +446,6 @@ public inline fun StringBuilder.appendLine(value: String?): StringBuilder = appe
 public inline fun StringBuilder.appendLine(value: Any?): StringBuilder = append(value).appendLine()
 
 /** Appends [value] to this [StringBuilder], followed by a line feed character (`\n`). */
-@OptIn(ExperimentalStdlibApi::class)
 @SinceKotlin("1.4")
 @kotlin.internal.InlineOnly
 public inline fun StringBuilder.appendLine(value: CharArray): StringBuilder = append(value).appendLine()
diff --git a/libraries/stdlib/test/text/StringBuilderTest.kt b/libraries/stdlib/test/text/StringBuilderTest.kt
index 0341c7ac791..f237f61b1bd 100644
--- a/libraries/stdlib/test/text/StringBuilderTest.kt
+++ b/libraries/stdlib/test/text/StringBuilderTest.kt
@@ -351,14 +351,14 @@ class StringBuilderTest {
             assertEquals("MMMmy insertT CharSequence test", sb.toString())
             sb.insertRange(31, "_*#", 0, 1)
             assertEquals("MMMmy insertT CharSequence test_", sb.toString())
-            sb.insertRange(0, null as CharSequence?, 0, 2)
+            sb.insertRange(0, "null" as CharSequence, 0, 2)
             assertEquals("nuMMMmy insertT CharSequence test_", sb.toString())
 
             assertFailsWith<IndexOutOfBoundsException> { sb.insert(-1, "_" as CharSequence) }
             assertFailsWith<IndexOutOfBoundsException> { sb.insert(sb.length + 1, StringBuilder("_")) }
-            assertFails { sb.insertRange(0, null as CharSequence?, -1, 0) }
-            assertFails { sb.insertRange(0, null as CharSequence?, 0, 5) }
-            assertFails { sb.insertRange(0, null as CharSequence?, 2, 1) }
+            assertFails { sb.insertRange(0, "null" as CharSequence, -1, 0) }
+            assertFails { sb.insertRange(0, "null" as CharSequence, 0, 5) }
+            assertFails { sb.insertRange(0, "null" as CharSequence, 2, 1) }
         }
     }