DOM wrappers that are not retained from Dart should be collected.

Source/WebCore/bindings/dart/DartDOMData.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 33 matching lines @@
     , m_recursion(0)
 {
     if (!context)
         return;
 
     ASSERT(context->isDocument()); // WorkerContext is not supported yet.
     Document* document = static_cast<Document*>(context);
     m_domWindow = document->domWindow();
 }
 
+DartDOMData::~DartDOMData()
+{
+    ASSERT(m_domMap.isEmpty());
+}
+
 PassRefPtr<DartApplicationLoader> DartDOMData::applicationLoader()
 {
     return m_applicationLoader;
 }
 
 }