Implement type inference for await expressions in front_end.

pkg/front_end/testcases/inference/future_union_downwards_2.dart

 // Copyright (c) 2017, 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.
 
 /*@testedFeatures=inference*/
 library test;
 
 import 'dart:async';
 
 class MyFuture<T> implements Future<T> {
@@ skipping 16 matching lines @@
               3
             ]);
 Future<List<int>> g2() async {
   return /*@typeArgs=int*/ [3];
 }
 
 Future<List<int>> g3() async {
   return new /*@typeArgs=List<int>*/ MyFuture.value(
       /*@typeArgs=int*/ [3]);
 }
+
+main() {}