Remove support for attribute event listeners.

client/dom/generated/src/interface/EventSource.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 EventSource extends EventTarget {
 
   static final int CLOSED = 2;
 
   static final int CONNECTING = 0;
 
   static final int OPEN = 1;
 
   String get URL();
 
-  EventListener get onerror();
-
-  void set onerror(EventListener value);
-
-  EventListener get onmessage();
-
-  void set onmessage(EventListener value);
-
-  EventListener get onopen();
-
-  void set onopen(EventListener value);
-
   int get readyState();
 
   void addEventListener(String type, EventListener listener, [bool useCapture]);
 
   void close();
 
   bool dispatchEvent(Event evt);
 
   void removeEventListener(String type, EventListener listener, [bool useCapture]);
 }