Merge the Sky Engine changes from the SkyDart branch

sky/engine/core/events/EventListener.h

Index: sky/engine/core/events/EventListener.h
diff --git a/sky/engine/core/events/EventListener.h b/sky/engine/core/events/EventListener.h
index e32a17a1de389bfafdc3d5b2fddefa66013c4188..38c55aefe308a36d080e17f0cfab6d825b30b8ac 100644
--- a/sky/engine/core/events/EventListener.h
+++ b/sky/engine/core/events/EventListener.h
@@ -23,8 +23,10 @@
 
 #include "sky/engine/wtf/RefCounted.h"
 
-namespace blink {
+typedef struct _Dart_WeakReferenceSet* Dart_WeakReferenceSet;
 
+namespace blink {
+class DartGCVisitor;
 class Event;
 class ExecutionContext;
 
@@ -34,6 +36,9 @@ public:
     virtual bool operator==(const EventListener&) = 0;
     virtual void handleEvent(ExecutionContext*, Event*) = 0;
 
+    virtual void AcceptDartGCVisitor(DartGCVisitor& visitor) const = 0;
+
+
 protected:
     explicit EventListener()
     {