Return futures on Stream.cancel when possible.

sdk/lib/async/stream.dart

Index: sdk/lib/async/stream.dart
diff --git a/sdk/lib/async/stream.dart b/sdk/lib/async/stream.dart
index 7661d9ffe99978261371223a41a66b8f9833d8a8..f4abf9aac9c2cda45b9b6da83c06c4f434724620 100644
--- a/sdk/lib/async/stream.dart
+++ b/sdk/lib/async/stream.dart
@@ -232,6 +232,7 @@ abstract class Stream<T> {
         onCancel: () {
           if (timer != null) timer.cancel();
           timer = null;
+          return Future._nullFuture;
         });
     return controller.stream;
   }
@@ -441,7 +442,7 @@ abstract class Stream<T> {
         onListen: onListen,
         onPause: () { subscription.pause(); },
         onResume: () { subscription.resume(); },
-        onCancel: () { subscription.cancel(); },
+        onCancel: () => subscription.cancel(),
         sync: true
       );
     }
@@ -499,7 +500,7 @@ abstract class Stream<T> {
         onListen: onListen,
         onPause: () { subscription.pause(); },
         onResume: () { subscription.resume(); },
-        onCancel: () { subscription.cancel(); },
+        onCancel: () => subscription.cancel(),
         sync: true
       );
     }
@@ -1407,7 +1408,10 @@ abstract class StreamSubscription<T> {
    * the subscription is canceled.
    *
    * Returns a future that is completed once the stream has finished
-   * its cleanup. May also return `null` if no cleanup was necessary.
+   * its cleanup.
+   *
+   * For historical reasons, may also return `null` if no cleanup was necessary.
+   * Returning `null` is deprecated and should be avoided.
    *
    * Typically, futures are returned when the stream needs to release resources.
    * For example, a stream might need to close an open file (as an asynchronous
@@ -1711,7 +1715,7 @@ abstract class StreamTransformer<S, T> {
    *             },
    *             onPause: () { subscription.pause(); },
    *             onResume: () { subscription.resume(); },
-   *             onCancel: () { subscription.cancel(); },
+   *             onCancel: () => subscription.cancel(),
    *             sync: true);
    *           return controller.stream.listen(null);
    *         });