Forward cancel subscription in stream pipes.

sdk/lib/async/stream_pipe.dart

 // 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 dart.async;
 
 /** Runs user code and takes actions depending on success or failure. */
 _runUserCode(userCode(),
              onSuccess(value),
              onError(error, StackTrace stackTrace)) {
@@ skipping 143 matching lines @@
 
   void _onResume() {
     if (_subscription == null) return;
     _subscription.resume();
   }
 
   Future _onCancel() {
     if (_subscription != null) {
       StreamSubscription subscription = _subscription;
       _subscription = null;
-      subscription.cancel();
+      return subscription.cancel();
     }
     return null;
   }
 
   // Methods used as listener on source subscription.
 
   void _handleData(S data) {
     _stream._handleData(data, this);
   }
 
@@ skipping 307 matching lines @@
         _addErrorWithReplacement(sink, e, s);
         return null;
       }
       if (!isEqual) {
         sink._add(inputEvent);
         _previous = inputEvent;
       }
     }
   }
 }