Use 'field' syntax in dart:dom interfaces.

client/dom/generated/src/interface/DedicatedWorkerContext.dart

 // Copyright (c) 2011, 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.
 
 // WARNING: Do not edit - generated code.
 
 interface DedicatedWorkerGlobalScope extends WorkerContext {
 
-  EventListener get onmessage();
-
-  void set onmessage(EventListener value);
+  EventListener onmessage;
 
   void postMessage(Object message, [List messagePorts]);
 
   void webkitPostMessage(Object message, [List transferList]);
 }
 
 interface DedicatedWorkerContext extends DedicatedWorkerGlobalScope {
 }