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

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

Index: client/html/generated/html/dartium/Notification.dart
diff --git a/client/html/generated/html/dartium/Notification.dart b/client/html/generated/html/dartium/Notification.dart
index c19f8dce50427592158707a669243cf9505f594f..5839d4e3e012bbae152c6c3aacaf445a8e45be28 100644
--- a/client/html/generated/html/dartium/Notification.dart
+++ b/client/html/generated/html/dartium/Notification.dart
@@ -2,6 +2,11 @@
 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); }
@@ -10,11 +15,6 @@ class _NotificationImpl extends _EventTargetImpl implements Notification {
 
   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;