[esnext] implement String padding proposal

src/js/harmony-string-padding.js

Index: src/js/harmony-string-padding.js
diff --git a/src/js/harmony-string-padding.js b/src/js/harmony-string-padding.js
new file mode 100644
index 0000000000000000000000000000000000000000..a6c6c474de5503e269b451b3fe89b1173e973453
--- /dev/null
+++ b/src/js/harmony-string-padding.js
@@ -0,0 +1,77 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+(function(global, utils) {
+
+%CheckIsBootstrapping();
+
+// -------------------------------------------------------------------
+// Imports
+
+var GlobalString = global.String;
+var MakeTypeError;
+
+utils.Import(function(from) {
+  MakeTypeError = from.MakeTypeError;
+});
+
+// -------------------------------------------------------------------
+// http://tc39.github.io/proposal-string-pad-start-end/
+
+function StringPad(thisString, maxLength, fillString) {
+  maxLength = TO_LENGTH(maxLength);
+  var stringLength = thisString.length;
+
+  if (maxLength <= stringLength) return "";
+
+  if (IS_UNDEFINED(fillString)) {
+    fillString = " ";
+  } else {
+    fillString = TO_STRING(fillString);
+    if (fillString === "") {
+      fillString = " ";
+    }
+  }
+
+  var fillLength = maxLength - stringLength;
+  var repetitions = (fillLength / fillString.length) | 0;
+  var remainingChars = (fillLength - fillString.length * repetitions) | 0;
+
+  var filler = "";
+  while (true) {
+    if (repetitions & 1) filler += fillString;
+    repetitions >>= 1;
+    if (repetitions === 0) break;
+    fillString += fillString;
+  }
+
+  if (remainingChars) {
+    filler += %_SubString(fillString, 0, remainingChars);
+  }
+
+  return filler;
+}
+
+function StringPadStart(maxLength, fillString) {
+  CHECK_OBJECT_COERCIBLE(this, "String.prototype.padStart")
+  var thisString = TO_STRING(this);
+
+  return StringPad(thisString, maxLength, fillString) + thisString;
+}
+%FunctionSetLength(StringPadStart, 1);
+
+function StringPadEnd(maxLength, fillString) {
+  CHECK_OBJECT_COERCIBLE(this, "String.prototype.padEnd")
+  var thisString = TO_STRING(this);
+
+  return thisString + StringPad(thisString, maxLength, fillString);
+}
+%FunctionSetLength(StringPadEnd, 1);
+
+utils.InstallFunctions(GlobalString.prototype, DONT_ENUM, [
+  "padStart", StringPadStart,
+  "padEnd", StringPadEnd
+]);
+
+});