[Wasm] Improve string constructor in stdlib

libraries/stdlib/wasm/src/kotlin/Text.kt

@@ -252,7 +252,15 @@ public actual fun Char.isWhitespace(): Boolean = TODO("Wasm stdlib: Text")
  * Converts the characters in the specified array to a string.
  */
 @SinceKotlin("1.2")
-public actual fun String(chars: CharArray): String = TODO("Wasm stdlib: Text")
+@Deprecated("Use CharArray.concatToString() instead", ReplaceWith("chars.concatToString()"))
+@DeprecatedSinceKotlin(warningSince = "1.4", errorSince = "1.5")
+public actual fun String(chars: CharArray): String {
+    var result = ""
+    for (char in chars) {
+        result += char
+    }
+    return result
+}
 
 /**
  * Converts the characters from a portion of the specified array to a string.
@@ -261,7 +269,17 @@ public actual fun String(chars: CharArray): String = TODO("Wasm stdlib: Text")
  * or `offset + length` is out of [chars] array bounds.
  */
 @SinceKotlin("1.2")
-public actual fun String(chars: CharArray, offset: Int, length: Int): String = TODO("Wasm stdlib: Text")
+@Deprecated("Use CharArray.concatToString(startIndex, endIndex) instead", ReplaceWith("chars.concatToString(offset, offset + length)"))
+@DeprecatedSinceKotlin(warningSince = "1.4", errorSince = "1.5")
+public actual fun String(chars: CharArray, offset: Int, length: Int): String {
+    if (offset < 0 || length < 0 || chars.size - offset < length)
+        throw IndexOutOfBoundsException("size: ${chars.size}; offset: $offset; length: $length")
+    var result = ""
+    for (index in offset until offset + length) {
+        result += chars[index]
+    }
+    return result
+}
 
 /**
  * Concatenates characters in this [CharArray] into a String.

libraries/stdlib/wasm/src/kotlin/text/StringBuilder.kt

@@ -15,7 +15,7 @@ internal fun insertString(array: CharArray, distIndex: Int, value: String, sourc
 }
 
 internal fun unsafeStringFromCharArray(array: CharArray, start: Int, size: Int): String =
-    kotlin.String(array.copyOfRange(start, start + size))
+    String.unsafeFromCharArray(array.copyOfRange(start, start + size))
 
 internal fun insertInt(array: CharArray, start: Int, value: Int): Int {
     val valueString = value.toString()