Make support for async/await syntax optional in the frontend.

pkg/compiler/lib/src/serialization/modelz.dart

 // Copyright (c) 2015, 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.
 
 /// Implementation of the element model used for deserialiation.
 ///
 /// These classes are created by [ElementDeserializer] triggered by the
 /// [Deserializer].
 
 library dart2js.serialization.modelz;
@@ skipping 479 matching lines @@
     if (_file == null) {
       throw new UnsupportedError('ScriptZ.file');
     }
     return _file;
   }
 
   void set file(SourceFile value) {
     _file = value;
   }
 
+  // TODO(johnniwinther): Decide if it is meaningful to serialize erroneous
+  // elements.
   @override
-  bool get isSynthesized => throw new UnsupportedError('ScriptZ.isSynthesized');
+  bool get isSynthesized => false;
 
   @override
   String get name {
     if (_file != null) {
       return _file.filename;
     }
     return resourceUri.toString();
   }
 
   // TODO(johnniwinther): Support the distinction between [readableUri] and
@@ skipping 1497 matching lines @@
   }
 
   @override
   ElementKind get kind => ElementKind.PREFIX;
 
   @override
   Element lookupLocalMember(String memberName) {
     return _unsupported('lookupLocalMember');
   }
 }