Move Appendable to its own file

libraries/stdlib/js/src/kotlin/text/AppendableJs.kt

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlin.text
+
+/**
+ * An object to which char sequences and values can be appended.
+ */
+public actual interface Appendable {
+    /**
+     * Appends the specified character [c] to this Appendable and returns this instance.
+     *
+     * @param c the character to append.
+     */
+    public actual fun append(c: Char): Appendable
+
+    /**
+     * Appends the specified character sequence [csq] to this Appendable and returns this instance.
+     *
+     * @param csq the character sequence to append. If [csq] is `null`, then the four characters `"null"` are appended to this Appendable.
+     */
+    public actual fun append(csq: CharSequence?): Appendable
+
+    /**
+     * Appends a subsequence of the specified character sequence [csq] to this Appendable and returns this instance.
+     *
+     * @param csq the character sequence from which a subsequence is appended. If [csq] is `null`,
+     *  then characters are appended as if [csq] contained the four characters `"null"`.
+     * @param start the beginning (inclusive) of the subsequence to append.
+     * @param end the end (exclusive) of the subsequence to append.
+     *
+     * @throws IndexOutOfBoundsException or [IllegalArgumentException] when [start] or [end] is out of range of the [csq] character sequence indices or when `start > end`.
+     */
+    public actual fun append(csq: CharSequence?, start: Int, end: Int): Appendable
+}

libraries/stdlib/js/src/kotlin/text/StringBuilderJs.kt

@@ -5,13 +5,6 @@
 
 package kotlin.text
 
-
-public actual interface Appendable {
-    public actual fun append(csq: CharSequence?): Appendable
-    public actual fun append(csq: CharSequence?, start: Int, end: Int): Appendable
-    public actual fun append(c: Char): Appendable
-}
-
 public actual class StringBuilder(content: String = "") : Appendable, CharSequence {
     actual constructor(capacity: Int) : this() {}