Update ObjectToString to Harmony-draft algorithm

src/promise.js

 // Copyright 2012 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:
 // var $Object = global.Object
 // var $WeakMap = global.WeakMap
@@ skipping 347 matching lines @@
   PromiseHasUserDefinedRejectHandler =
       function PromiseHasUserDefinedRejectHandler() {
     return PromiseHasUserDefinedRejectHandlerRecursive(this);
   };
 
   // -------------------------------------------------------------------
   // Install exported functions.
 
   %CheckIsBootstrapping();
   %AddNamedProperty(global, 'Promise', $Promise, DONT_ENUM);
+  %AddNamedProperty(
+      $Promise.prototype, symbolToStringTag, "Promise", DONT_ENUM | READ_ONLY);
   InstallFunctions($Promise, DONT_ENUM, [
     "defer", PromiseDeferred,
     "accept", PromiseResolved,
     "reject", PromiseRejected,
     "all", PromiseAll,
     "race", PromiseOne,
     "resolve", PromiseCast
   ]);
   InstallFunctions($Promise.prototype, DONT_ENUM, [
     "chain", PromiseChain,
     "then", PromiseThen,
     "catch", PromiseCatch
   ]);
 
 })();