Adding support for the MouseWheel event in Streams.

tools/dom/src/EventStreamProvider.dart

 // Copyright (c) 2013, 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;
 
 /**
  * Adapter for exposing DOM events as Dart streams.
  */
 class _EventStream<T extends Event> extends Stream<T> {
@@ skipping 117 matching lines @@
    *     MediaElement.pauseEvent.forTarget(document.body).listen(...);
    *
    * See also:
    *
    * [addEventListener](http://docs.webplatform.org/wiki/dom/methods/addEventListener)
    */
   Stream<T> forTarget(EventTarget e, {bool useCapture: false}) {
     return new _EventStream(e, _eventType, useCapture);
   }
 }
+
+/**
+ * A factory to expose DOM events as streams, where the DOM event name has to
+ * be determined on the fly (for example, mouse wheel events).
+ */
+class _CustomEventStreamProvider<T extends Event>
+    implements EventStreamProvider<T> {
+
+  final _eventTypeGetter;
+  const _CustomEventStreamProvider(this._eventTypeGetter);
+
+  Stream<T> forTarget(EventTarget e, {bool useCapture: false}) {
+    return new _EventStream(e, _eventTypeGetter(e), useCapture);
+  }
+}