Moved most of html lib generating scripts into tools.

tools/dom/templates/html/dart2js/impl_ElementEvents.darttemplate

 // 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.
 
 part of html;
 
 class $CLASSNAME extends $SUPER {
   $CLASSNAME(EventTarget _ptr) : super(_ptr);
 $!MEMBERS
   EventListenerList get mouseWheel {
     if (JS('bool', '#.onwheel !== undefined', _ptr)) {
       // W3C spec, and should be IE9+, but IE has a bug exposing onwheel.
       return this['wheel'];
     } else if (JS('bool', '#.onmousewheel !== undefined', _ptr)) {
       // Chrome & IE
       return this['mousewheel'];
     } else {
       // Firefox
       return this['DOMMouseScroll'];
     }
   }
 }