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

client/html/generated/html/interface/BodyElement.dart

 // 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.
 
 // WARNING: Do not edit - generated code.
 
 interface BodyElement extends Element {
 
+  BodyElementEvents get on();
+
   String aLink;
 
   String background;
 
   String bgColor;
 
   String link;
 
   String vLink;
-
-  BodyElementEvents get on();
 }
 
 interface BodyElementEvents extends ElementEvents {
 
   EventListenerList get beforeUnload();
 
   EventListenerList get blur();
 
   EventListenerList get error();
 
@@ skipping 10 matching lines @@
   EventListenerList get online();
 
   EventListenerList get popState();
 
   EventListenerList get resize();
 
   EventListenerList get storage();
 
   EventListenerList get unload();
 }