Return futures on Stream.cancel when possible.

sdk/lib/async/stream_impl.dart

Index: sdk/lib/async/stream_impl.dart
diff --git a/sdk/lib/async/stream_impl.dart b/sdk/lib/async/stream_impl.dart
index 8c9af64bb9e01fd4bfe9511e9024117bcce3e8e8..680ed0ef1ad9350c72882a0629b39373337fcccb 100644
--- a/sdk/lib/async/stream_impl.dart
+++ b/sdk/lib/async/stream_impl.dart
@@ -778,7 +778,7 @@ class _DoneStreamSubscription<T> implements StreamSubscription<T> {
     }
   }
 
-  Future cancel() => null;
+  Future cancel() => new Future.value(null);

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

Future._nullFuture

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

Done.

 
   Future/*<E>*/ asFuture/*<E>*/([var/*=E*/ futureValue]) {
     _Future/*<E>*/ result = new _Future/*<E>*/();
@@ -916,7 +916,7 @@ class _BroadcastSubscriptionWrapper<T> implements StreamSubscription<T> {
 
   Future cancel() {
     _stream._cancelSubscription();
-    return null;
+    return new Future.value(null);

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

Future._nullFuture
(I will stop saying it now, but do change the rest too :)

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

Done.

   }
 
   bool get isPaused {
@@ -1032,7 +1032,7 @@ class _StreamIteratorImpl<T> implements StreamIterator<T> {
 
   Future cancel() {
     StreamSubscription subscription = _subscription;
-    if (subscription == null) return null;
+    if (subscription == null) return new Future.value(null);
     if (_state == _STATE_MOVING) {
       _Future<bool> hasNext = _futureOrPrefetch as Object /*=_Future<bool>*/;
       _clear();