[promise] Spec compliance fixes for async/await

test/mjsunit/harmony/async-await-no-constructor.js

+// 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.
+
+// Flags: --harmony-async-await --allow-natives-syntax
+
+'use strict';
+
+var resolved = Promise.resolve();
+var count = 0;
+
+Object.defineProperty(Promise.prototype, 'constructor',
+                      { get() { count++; return Promise; } })
+
+async function foo() {
+  await resolved;
+  return resolved;
+}
+
+foo();
+%RunMicrotasks();
+assertEquals(0, count);