Merge the Sky Engine changes from the SkyDart branch

sky/engine/core/events/EventListener.h

 /*
  * Copyright (C) 2006, 2008, 2009 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  *
  */
 
 #ifndef SKY_ENGINE_CORE_EVENTS_EVENTLISTENER_H_
 #define SKY_ENGINE_CORE_EVENTS_EVENTLISTENER_H_
 
 #include "sky/engine/wtf/RefCounted.h"
 
+typedef struct _Dart_WeakReferenceSet* Dart_WeakReferenceSet;
+
 namespace blink {
-
+class DartGCVisitor;
 class Event;
 class ExecutionContext;
 
 class EventListener : public RefCounted<EventListener> {
 public:
     virtual ~EventListener() { }
     virtual bool operator==(const EventListener&) = 0;
     virtual void handleEvent(ExecutionContext*, Event*) = 0;
 
+    virtual void AcceptDartGCVisitor(DartGCVisitor& visitor) const = 0;
+
+
 protected:
     explicit EventListener()
     {
     }
 };
 
 }
 
 #endif  // SKY_ENGINE_CORE_EVENTS_EVENTLISTENER_H_