[es6] Align Promise.resolve with the spec

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.
 
 (function(global, utils, extrasUtils) {
 
 "use strict";
 
 %CheckIsBootstrapping();
 
@@ skipping 267 matching lines @@
       }
     },
     onReject,
     PromiseChain
   );
 }
 
 // Combinators.
 
 function PromiseCast(x) {
-  // TODO(rossberg): cannot do better until we support @@create.
-  return IsPromise(x) ? x : new this(function(resolve) { resolve(x) });
+  if (IsPromise(x) && x.constructor === this) {
+    return x;
+  } else {
+    return new this(function(resolve) { resolve(x) });
+  }
 }
 
 function PromiseAll(iterable) {
   var deferred = %_CallFunction(this, PromiseDeferred);
   var resolutions = [];
   try {
     var count = 0;
     var i = 0;
     for (var value of iterable) {
       this.resolve(value).then(
@@ skipping 91 matching lines @@
 // This allows extras to create promises quickly without building extra
 // resolve/reject closures, and allows them to later resolve and reject any
 // promise without having to hold on to those closures forever.
 utils.InstallFunctions(extrasUtils, 0, [
   "createPromise", PromiseCreate,
   "resolvePromise", PromiseResolve,
   "rejectPromise", PromiseReject
 ]);
 
 })