Return futures on Stream.cancel when possible.

sdk/lib/async/stream_controller.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;
 
 // -------------------------------------------------------------------
 // Controller for creating and adding events to a stream.
 // -------------------------------------------------------------------
 
@@ skipping 691 matching lines @@
       _StreamControllerAddStreamState<T> addState =
           _varData as Object /*=_StreamControllerAddStreamState<T>*/;
       result = addState.cancel();
     }
     _varData = null;
     _state =
         (_state & ~(_STATE_SUBSCRIBED | _STATE_ADDSTREAM)) | _STATE_CANCELED;
 
     if (onCancel != null) {
       if (result == null) {
-        // Only introduce a future if one is needed.
-        // If _onCancel returns null, no future is needed.
+        // Introduce a future if none was returned.
         try {
-          result = onCancel();
+          result = onCancel() ?? new Future.value(null);

[Lasse Reichstein Nielsen, 2016/08/01 15:23:26]

Use `Future._nullFuture`.

[floitsch, 2016/08/01 21:00:37]

Moved the 'Future._nullFuture' to the `cancel` function, but done.

         } catch (e, s) {
           // Return the error in the returned future.
           // Complete it asynchronously, so there is time for a listener
           // to handle the error.
           result = new _Future().._asyncCompleteError(e, s);
         }
       } else {
         // Simpler case when we already know that we will return a future.
         result = result.whenComplete(onCancel);
       }
@@ skipping 213 matching lines @@
   _StreamControllerAddStreamState(_StreamController<T> controller,
                                   this.varData,
                                   Stream source,
                                   bool cancelOnError)
       : super(controller, source, cancelOnError) {
     if (controller.isPaused) {
       addSubscription.pause();
     }
   }
 }