Update ObjectToString to Harmony-draft algorithm

src/harmony-tostring.js

+// Copyright 2013 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.
+
+'use strict';
+
+// This file relies on the fact that the following declaration has been made
+// in runtime.js and symbol.js:
+// var $Object = global.Object;
+// var $Symbol = global.Symbol;
+
+var symbolToStringTag = InternalSymbol("Symbol.toStringTag");
+
+var kBuiltinStringTags = {
+  "__proto__": null,
+  "Arguments": true,
+  "Array": true,
+  "Boolean": true,
+  "Date": true,
+  "Error": true,
+  "Function": true,
+  "Number": true,
+  "RegExp": true,
+  "String": true
+};
+
+DefaultObjectToString = ObjectToStringHarmony;
+// ES6 draft 08-24-14, section 19.1.3.6
+function ObjectToStringHarmony() {
+  if (IS_UNDEFINED(this) && !IS_UNDETECTABLE(this)) return "[object Undefined]";
+  if (IS_NULL(this)) return "[object Null]";
+  var O = ToObject(this);
+  var builtinTag = %_ClassOf(O);
+  var tag = O[symbolToStringTag];
+  if (IS_UNDEFINED(tag)) {
+    tag = builtinTag;
+  } else if (!IS_STRING(tag)) {
+    return "[object ???]"
+  } else if (tag !== builtinTag && kBuiltinStringTags[tag]) {
+    return "[object ~" + tag + "]";
+  }
+  return "[object " + tag + "]";
+}
+
+function HarmonyToStringExtendSymbolPrototype() {
+  %CheckIsBootstrapping();
+
+  InstallConstants($Symbol, $Array(
+    // TODO(dslomov, caitp): Move to symbol.js when shipping
+   "toStringTag", symbolToStringTag
+  ));
+}
+
+HarmonyToStringExtendSymbolPrototype();
+
+function HarmonyToStringExtendObjectPrototype() {
+  %CheckIsBootstrapping();
+
+  // Set up the non-enumerable functions on the Array prototype object.
+  var desc = ToPropertyDescriptor({
+    value: ObjectToStringHarmony
+  });
+  DefineOwnProperty($Object.prototype, "toString", desc, false);
+}
+
+HarmonyToStringExtendObjectPrototype();