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

client/html/generated/html/interface/Element.dart

 // Copyright (c) 2011, 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.
 
 /**
  * Provides a Map abstraction on top of data-* attributes, similar to the
  * dataSet in the old DOM.
  */
@@ skipping 279 matching lines @@
   Future<CSSStyleDeclaration> get computedStyle();
 
   /** @domName Window.getComputedStyle */
   Future<CSSStyleDeclaration> getComputedStyle(String pseudoElement);
 
   Element clone(bool deep);
 
   Element get parent();
 
 
+  ElementEvents get on();
+
   static final int ALLOW_KEYBOARD_INPUT = 1;
 
   final int _childElementCount;
 
   final HTMLCollection _children;
 
   final DOMTokenList classList;
 
   String _className;
 
@@ skipping 58 matching lines @@
   final String tagName;
 
   String title;
 
   bool translate;
 
   final String webkitRegionOverflow;
 
   String webkitdropzone;
 
-  ElementEvents get on();
-
   void blur();
 
   void click();
 
   void focus();
 
   String _getAttribute(String name);
 
   ClientRect _getBoundingClientRect();
 
@@ skipping 116 matching lines @@
   EventListenerList get touchEnd();
 
   EventListenerList get touchLeave();
 
   EventListenerList get touchMove();
 
   EventListenerList get touchStart();
 
   EventListenerList get transitionEnd();
 }