Adding support for the MouseWheel event in Streams.

tools/dom/templates/html/impl/impl_Element.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;
 
 // TODO(jacobr): use _Lists.dart to remove some of the duplicated
 // functionality.
 class _ChildrenElementList implements List {
   // Raw Element.
@@ skipping 709 matching lines @@
    *
    * [wc]: http://dvcs.w3.org/hg/webcomponents/raw-file/tip/explainer/index.html
    * [x-tags]: http://x-tags.org/
    */
 $if DART2JS
   @Creates('Null')  // Set from Dart code; does not instantiate a native type.
 $endif
   var xtag;
 
 $if DART2JS
+  static const EventStreamProvider<WheelEvent> mouseWheelEvent =
+      const _CustomEventStreamProvider<WheelEvent>(
+        Element._determineMouseWheelEventType);
+
+  static String _determineMouseWheelEventType(EventTarget e) {
+    if (JS('bool', '#.onwheel !== undefined', e)) {
+      // W3C spec, and should be IE9+, but IE has a bug exposing onwheel.
+      return 'wheel';
+    } else if (JS('bool', '#.onmousewheel !== undefined', e)) {
+      // Chrome & IE
+      return 'mousewheel';
+    } else {
+      // Firefox
+      return 'DOMMouseScroll';
+    }
+  }
+
   /**
    * Creates a text node and inserts it into the DOM at the specified location.
    *
    * To see the possible values for [where], read the doc for
    * [insertAdjacentHtml].
    *
    * See also:
    *
    * * [insertAdjacentHtml]
    */
@@ skipping 234 matching lines @@
 $if DART2JS
   // Optimization to improve performance until the dart2js compiler inlines this
   // method.
   static dynamic createElement_tag(String tag) =>
       JS('Element', 'document.createElement(#)', tag);
 $else
   static Element createElement_tag(String tag) =>
       document.$dom_createElement(tag);
 $endif
 }