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..cb2e20d0f1373c8ec59700d85e76e81e2dc91f5a 100644
--- a/sdk/lib/async/stream_impl.dart
+++ b/sdk/lib/async/stream_impl.dart
@@ -188,9 +188,10 @@ class _BufferingStreamSubscription<T> implements StreamSubscription<T>,
     // error or done event pending (waiting for the cancel to be done) discard
     // that event.
     _state &= ~_STATE_WAIT_FOR_CANCEL;
-    if (_isCanceled) return _cancelFuture;
-    _cancel();
-    return _cancelFuture;
+    if (!_isCanceled) {
+      _cancel();
+    }
+    return _cancelFuture ?? Future._nullFuture;
   }
 
   Future/*<E>*/ asFuture/*<E>*/([var/*=E*/ futureValue]) {
@@ -199,8 +200,14 @@ class _BufferingStreamSubscription<T> implements StreamSubscription<T>,
     // Overwrite the onDone and onError handlers.
     _onDone = () { result._complete(futureValue); };
     _onError = (error, stackTrace) {
-      cancel();
-      result._completeError(error, stackTrace);
+      Future cancelFuture = cancel();
+      if (!identical(cancelFuture, Future._nullFuture)) {
+        cancelFuture.whenComplete(() {
+          result._completeError(error, stackTrace);
+        });
+      } else {
+        result._completeError(error, stackTrace);
+      }
     };
 
     return result;
@@ -361,7 +368,8 @@ class _BufferingStreamSubscription<T> implements StreamSubscription<T>,
     if (_cancelOnError) {
       _state |= _STATE_WAIT_FOR_CANCEL;
       _cancel();
-      if (_cancelFuture is Future) {
+      if (_cancelFuture is Future &&
+          !identical(_cancelFuture, Future._nullFuture)) {
         _cancelFuture.whenComplete(sendError);
       } else {
         sendError();
@@ -389,7 +397,8 @@ class _BufferingStreamSubscription<T> implements StreamSubscription<T>,
 
     _cancel();
     _state |= _STATE_WAIT_FOR_CANCEL;
-    if (_cancelFuture is Future) {
+    if (_cancelFuture is Future &&
+        !identical(_cancelFuture, Future._nullFuture)) {
       _cancelFuture.whenComplete(sendDone);
     } else {
       sendDone();
@@ -778,7 +787,7 @@ class _DoneStreamSubscription<T> implements StreamSubscription<T> {
     }
   }
 
-  Future cancel() => null;
+  Future cancel() => Future._nullFuture;
 
   Future/*<E>*/ asFuture/*<E>*/([var/*=E*/ futureValue]) {
     _Future/*<E>*/ result = new _Future/*<E>*/();
@@ -916,7 +925,7 @@ class _BroadcastSubscriptionWrapper<T> implements StreamSubscription<T> {
 
   Future cancel() {
     _stream._cancelSubscription();
-    return null;
+    return Future._nullFuture;
   }
 
   bool get isPaused {
@@ -1032,7 +1041,7 @@ class _StreamIteratorImpl<T> implements StreamIterator<T> {
 
   Future cancel() {
     StreamSubscription subscription = _subscription;
-    if (subscription == null) return null;
+    if (subscription == null) return Future._nullFuture;
     if (_state == _STATE_MOVING) {
       _Future<bool> hasNext = _futureOrPrefetch as Object /*=_Future<bool>*/;
       _clear();