Return futures on Stream.cancel when possible.

sdk/lib/async/stream_transformers.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 dart.async;
 
 /**
  * Wraps an [_EventSink] so it exposes only the [EventSink] interface.
  */
 class _EventSinkWrapper<T> implements EventSink<T> {
@@ skipping 91 matching lines @@
   }
 
   void _onResume() {
     if (_isSubscribed) _subscription.resume();
   }
 
   Future _onCancel() {
     if (_isSubscribed) {
       StreamSubscription subscription = _subscription;
       _subscription = null;
-      subscription.cancel();
+      return subscription.cancel();
     }
     return null;
   }
 
   void _handleData(S data) {
     try {
       _transformerSink.add(data);
     } catch (e, s) {
       _addError(e, s);
     }
@@ skipping 181 matching lines @@
                                  void onDone(),
                                  bool cancelOnError }) {
     cancelOnError = identical(true, cancelOnError);
     StreamSubscription<T> result = _transformer(_stream, cancelOnError);
     result.onData(onData);
     result.onError(onError);
     result.onDone(onDone);
     return result;
   }
 }