Fix dartium checked mode build break.

client/dom/templates/html/dartium/impl_EventTarget.darttemplate

 // Copyright (c) 2012, 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.
 
 class _EventsImpl implements Events {
 
   final _EventTargetImpl _ptr;
 
   final Map<String, EventListenerList> _listenerMap;
 
@@ skipping 32 matching lines @@
 
   EventListenerList remove(EventListener listener, [bool useCapture = false]) {
     _remove(listener, useCapture);
     return this;
   }
 
   bool dispatch(Event evt) {
     // TODO(jacobr): what is the correct behavior here. We could alternately
     // force the event to have the expected type.
     assert(evt.type == _type);
-    return _ptr._dispatchEvent(_unwrap(evt));
+    return _ptr._dispatchEvent(evt);
   }
 
   void _add(EventListener listener, bool useCapture) {
     _ptr._addEventListener(_type,
                           _findOrAddWrapper(listener, useCapture),
                           useCapture);
   }
 
   void _remove(EventListener listener, bool useCapture) {
     Function wrapper = _removeWrapper(listener, useCapture);
@@ skipping 42 matching lines @@
 
   Events _on;
 
   Events get on() {
     if (_on == null) _on = new _EventsImpl(this);
     return _on;
   }
 
 $!MEMBERS
 }