Return futures on Stream.cancel when possible.

sdk/lib/async/stream_controller.dart

Index: sdk/lib/async/stream_controller.dart
diff --git a/sdk/lib/async/stream_controller.dart b/sdk/lib/async/stream_controller.dart
index 87b64e893d1e52dc89f7b55f5b3ca19175a0bba1..d6cc66784cd2ffd98eee403ea086900a9a4a750a 100644
--- a/sdk/lib/async/stream_controller.dart
+++ b/sdk/lib/async/stream_controller.dart
@@ -709,10 +709,9 @@ abstract class _StreamController<T> implements StreamController<T>,
 
     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