Implementation of async-await transformation on js ast.

tests/language/await_regression_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
 
 
 import 'dart:async';
 import 'package:expect/expect.dart';
 
 later(vodka) => new Future.value(vodka);
 
 manana(tequila) async => tequila;
 
 // Regression test for issue 21536.
 testNestedFunctions() async {
-  var a = await later('Asterix').then((tonic) {
+  var a = await (later('Asterix').then((tonic) {
     return later(tonic);
-  });
-  var o = await manana('Obelix').then(manana);
+  }));
+  var o = await (manana('Obelix').then(manana));
   Expect.equals("$a and $o", "Asterix and Obelix");
 }
 
 addLater({a, b}) => new Future.value(a + b);
 
 // Regression test for issue 21480.
 testNamedArguments() async {
   var sum = await addLater(a:5, b:10);
   Expect.equals(sum, 15);
   sum = await addLater(b:11, a:-11);
   Expect.equals(sum, 0);
 }
 
 main() async {
   testNestedFunctions();
   testNamedArguments();
 }