Don't expect a `null` return from subscription.cancel.

test/stream_group_test.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:async';
 
 import 'package:async/async.dart';
 import 'package:test/test.dart';
 
 main() {
@@ skipping 489 matching lines @@
       test("stops emitting events for a stream that's removed", () async {
         var controller = new StreamController<String>();
         streamGroup.add(controller.stream);
 
         expect(streamGroup.stream.toList(), completion(equals(["first"])));
 
         controller.add("first");
         await flushMicrotasks();
         controller.add("second");
 
-        expect(streamGroup.remove(controller.stream), isNull);
+        expect(streamGroup.remove(controller.stream), completion(null));
         expect(streamGroup.close(), completes);
       });
 
       test("is a no-op for an unknown stream", () {
         var controller = new StreamController<String>();
         expect(streamGroup.remove(controller.stream), isNull);
       });
 
       test("and closed closes the group when the last stream is removed",
           () async {
@@ skipping 20 matching lines @@
       test("doesn't emit events from a removed stream", () {
         var controller = new StreamController<String>();
         streamGroup.add(controller.stream);
 
         // The subscription to [controller.stream] is canceled synchronously, so
         // the first event is dropped even though it was added before the
         // removal. This is documented in [StreamGroup.remove].
         expect(streamGroup.stream.toList(), completion(isEmpty));
 
         controller.add("first");
-        expect(streamGroup.remove(controller.stream), isNull);
+        expect(streamGroup.remove(controller.stream), completion(null));
         controller.add("second");
 
         expect(streamGroup.close(), completes);
       });
 
       test("cancels the stream's subscription", () async {
         var controller = new StreamController<String>();
         streamGroup.add(controller.stream);
 
         streamGroup.stream.listen(null);
@@ skipping 150 matching lines @@
       controller.close();
 
       await streamGroup.close();
       expect(events, equals(["one", "two", "three", "four", "five", "six"]));
     });
   });
 }
 
 /// Wait for all microtasks to complete.
 Future flushMicrotasks() => new Future.delayed(Duration.ZERO);