Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

 // Copyright (c) 2013, 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 elements.modelx;
 
 import 'elements.dart';
 import '../constants/expressions.dart';
 import '../helpers/helpers.dart';  // Included for debug helpers.
 import '../tree/tree.dart';
@@ skipping 269 matching lines @@
   bool get isTopLevel => false;
 
   bool get isSynthesized => true;
 
   AbstractFieldElement abstractField;
 
   unsupported() {
     throw 'unsupported operation on erroneous element';
   }
 
+  get asyncMarker => AsyncMarker.SYNC;
   Link<MetadataAnnotation> get metadata => unsupported();
   bool get hasNode => false;
   get node => unsupported();
   get hasResolvedAst => false;
   get resolvedAst => unsupported();
   get type => unsupported();
   get cachedNode => unsupported();
   get functionSignature => unsupported();
   get patch => null;
   get origin => this;
@@ skipping 1233 matching lines @@
   final Modifiers modifiers;
 
   List<FunctionElement> nestedClosures = new List<FunctionElement>();
 
   FunctionSignature functionSignatureCache;
 
   final bool _hasNoBody;
 
   AbstractFieldElement abstractField;
 
+  AsyncMarker asyncMarker = AsyncMarker.SYNC;
+
   BaseFunctionElementX(String name,
                        ElementKind kind,
                        Modifiers this.modifiers,
                        Element enclosing,
                        bool hasNoBody)
       : super(name, kind, enclosing),
         _hasNoBody = hasNoBody {
     assert(modifiers != null);
   }
 
@@ skipping 1156 matching lines @@
   AstElement get definingElement;
 
   bool get hasResolvedAst => definingElement.hasTreeElements;
 
   ResolvedAst get resolvedAst {
     return new ResolvedAst(declaration,
         definingElement.node, definingElement.treeElements);
   }
 
 }