Refactor the event-generation code to locate it in systemhtml.py.

client/html/generated/html/dartium/Notification.dart

 
 class _NotificationImpl extends _EventTargetImpl implements Notification {
   _NotificationImpl._wrap(ptr) : super._wrap(ptr);
 
+  _NotificationEventsImpl get on() {
+    if (_on == null) _on = new _NotificationEventsImpl(this);
+    return _on;
+  }
+
   String get dir() => _wrap(_ptr.dir);
 
   void set dir(String value) { _ptr.dir = _unwrap(value); }
 
   String get replaceId() => _wrap(_ptr.replaceId);
 
   void set replaceId(String value) { _ptr.replaceId = _unwrap(value); }
 
-  _NotificationEventsImpl get on() {
-    if (_on == null) _on = new _NotificationEventsImpl(this);
-    return _on;
-  }
-
   void cancel() {
     _ptr.cancel();
     return;
   }
 
   void show() {
     _ptr.show();
     return;
   }
 }
 
 class _NotificationEventsImpl extends _EventsImpl implements NotificationEvents {
   _NotificationEventsImpl(_ptr) : super(_ptr);
 
   EventListenerList get click() => _get('click');
 
   EventListenerList get close() => _get('close');
 
   EventListenerList get display() => _get('display');
 
   EventListenerList get error() => _get('error');
 
   EventListenerList get show() => _get('show');
 }