Clean up WheelEvent

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 $LIBRARYNAME;
 
 @deprecated
 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'];
-    }
-  }
+  EventListenerList get mouseWheel => this['wheel'];
 }