Fix dartium checked mode build break.

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

Index: client/dom/templates/html/dartium/impl_EventTarget.darttemplate
diff --git a/client/dom/templates/html/dartium/impl_EventTarget.darttemplate b/client/dom/templates/html/dartium/impl_EventTarget.darttemplate
index 6cad86779008d9006e84346ce5be3b6a8abf635f..651758c0b03542666d7f4b04faddf7183644624e 100644
--- a/client/dom/templates/html/dartium/impl_EventTarget.darttemplate
+++ b/client/dom/templates/html/dartium/impl_EventTarget.darttemplate
@@ -50,7 +50,7 @@ class _EventListenerListImpl implements EventListenerList {
     // 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) {