Fix build breakage caused by commit 5196.

client/html/frog/html_frog.dart

 #library('html');
 
 // 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.
 
 // DO NOT EDIT
 // Auto-generated dart:html library.
 
 
@@ skipping 4831 matching lines @@
 
   EventListenerList get touchEnd() => _get('touchend');
 
   EventListenerList get touchMove() => _get('touchmove');
 
   EventListenerList get touchStart() => _get('touchstart');
 }
 
 class _DocumentFragmentImpl extends _NodeImpl implements DocumentFragment native "*DocumentFragment" {
 
-  ElementEvents get on() =>
-    new ElementEvents(this);
+  _ElementEventsImpl get on() =>
+    new _ElementEventsImpl(this);
 
   _ElementImpl query(String selectors) native "return this.querySelector(selectors);";
 
   _NodeListImpl _querySelectorAll(String selectors) native "return this.querySelectorAll(selectors);";
 }
 
 class _DocumentTypeImpl extends _NodeImpl implements DocumentType native "*DocumentType" {
 
   final _NamedNodeMapImpl entities;
 
@@ skipping 26108 matching lines @@
     if (length < 0) throw new IllegalArgumentException('length');
     if (start < 0) throw new IndexOutOfRangeException(start);
     int end = start + length;
     if (end > a.length) throw new IndexOutOfRangeException(end);
     for (int i = start; i < end; i++) {
       accumulator.add(a[i]);
     }
     return accumulator;
   }
 }