[JS IR BE] support CharSequence and String methods

libraries/stdlib/js/irRuntime/charSequence.kt

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. 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.js
+
+internal annotation class DoNotIntrinsify
+
+@PublishedApi
+@DoNotIntrinsify
+internal fun charSequenceGet(a: CharSequence, index: Int): Char {
+    return  if (a is String) {
+        Char(a.asDynamic().charCodeAt(index).unsafeCast<Int>())
+    } else {
+        a[index]
+    }
+}
+
+@PublishedApi
+@DoNotIntrinsify
+internal fun charSequenceLength(a: CharSequence): Int {
+    return if (a is String) {
+        a.asDynamic().length.unsafeCast<Int>()
+    } else {
+        a.length
+    }
+}
+
+@PublishedApi
+@DoNotIntrinsify
+internal fun charSequenceSubSequence(a: CharSequence, startIndex: Int, endIndex: Int): CharSequence {
+    return if (a is String) {
+        a.asDynamic().substring(startIndex, endIndex).unsafeCast<String>()
+    } else {
+        a.subSequence(startIndex, endIndex)
+    }
+}