First pass at cleaning up html lib warnings from the analyzer.

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;
 
 /**
  * A factory to expose DOM events as Streams.
  */
 class EventStreamProvider<T extends Event> {
@@ skipping 205 matching lines @@
   EventTarget _target;
   final String _eventType;
   var _onData;
   final bool _useCapture;
 
   _EventStreamSubscription(this._target, this._eventType, onData,
       this._useCapture) : _onData = _wrapZone(onData) {
     _tryResume();
   }
 
-  void cancel() {
-    if (_canceled) return;
+  Future cancel() {
+    if (_canceled) return null;
 
     _unlisten();
     // Clear out the target to indicate this is complete.
     _target = null;
     _onData = null;
+    return null;
   }
 
   bool get _canceled => _target == null;
 
   void onData(void handleData(T event)) {
     if (_canceled) {
       throw new StateError("Subscription has been canceled.");
     }
     // Remove current event listener.
     _unlisten();
@@ skipping 178 matching lines @@
 
   ElementStream<T> _forElementList(ElementList e,
       {bool useCapture: false}) {
     return new _ElementListEventStreamImpl(e, _eventTypeGetter(e), useCapture);
   }
 
   String getEventType(EventTarget target) {
     return _eventTypeGetter(target);
   }
 }