Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/resolution/resolution.dart

 // Copyright (c) 2012, 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.
 
 library resolution;
 
 import 'dart:collection' show Queue;
 
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 import '../dart_backend/dart_backend.dart' show DartBackend;
 import '../dart_types.dart';
 import '../dart2jslib.dart';
 import '../tree/tree.dart';
 import '../elements/elements.dart';
 import '../elements/modelx.dart'
     show BaseClassElementX,
+         BaseFunctionElementX,
          ConstructorElementX,
          ErroneousElementX,
          FieldElementX,
          FormalElementX,
          FunctionElementX,
          FunctionSignatureX,
          InitializingFormalElementX,
          LabelDefinitionX,
          LocalFunctionElementX,
          LocalParameterElementX,
@@ skipping 11 matching lines @@
 import '../ordered_typeset.dart' show OrderedTypeSet, OrderedTypeSetBuilder;
 import '../util/util.dart';
 
 import 'class_members.dart' show MembersCreator;
 import 'secret_tree_element.dart' show getTreeElement, setTreeElement;
 
 part 'members.dart';
 part 'registry.dart';
 part 'scope.dart';
 part 'signatures.dart';