Class wrapping window's before closing event in HandlerRegistration manner.

class ClosingHandlerRegistration implements HandlerRegistration {
 
 ClosingHandler _handler;
 dart_async.StreamSubscription listener;
 
 ClosingHandlerRegistration(_handler) {
   listener = dart_html.window.onBeforeUnload.listen(_onData);
 }
 
 void _onData(dart_html.Event evt) {
   ClosingEvent event = new ClosingEvent();
   _handler.onWindowClosing(event);
 }
 
 /**
  * Deregisters the handler associated with this registration object if the
  * handler is still attached to the event source. If the handler is no longer
  * attached to the event source, this is a no-op.
  */
 void removeHandler() {
   listener.cancel();
 }
}