Implement type inference for await expressions in front_end.

pkg/front_end/testcases/inference/future_union_async_conditional_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 11 matching lines @@
 
 Future<int> g2(bool x) async =>
     /*info:DOWN_CAST_COMPOSITE*/ x
         ? 42
         : new /*@typeArgs=int*/ MyFuture.value(42);
 Future<int> g3(bool x) async {
   var /*@type=Object*/ y =
       x ? 42 : new /*@typeArgs=dynamic*/ MyFuture.value(42);
   return /*info:DOWN_CAST_COMPOSITE*/ y;
 }
+
+main() {}