Clear EventHandler on Stop, to avoid future usages.

runtime/bin/eventhandler_patch.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.
 
 import 'dart:nativewrappers';
 
 patch class _EventHandler {
   /* patch */ static void _start() {
     if (_eventHandler == null) {
       _eventHandler = new _EventHandlerImpl();
       _eventHandler._start();
     }
   }
 
   /* patch */ static _sendData(Object sender,
                                ReceivePort receivePort,
                                int data) {
     if (_eventHandler != null) {
       _eventHandler._sendData(sender, receivePort, data);
     }
   }
 
   static _EventHandlerImpl _eventHandler;
 }
 
 
-class _EventHandlerImpl extends NativeFieldWrapperClass1 {
+class _EventHandlerImpl {
   _EventHandlerImpl() { }
   void _start() native "EventHandler_Start";
   void _sendData(Object sender, ReceivePort receivePort, int data)
       native "EventHandler_SendData";
 }