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

R=floitsch@google.com

Review URL: https://codereview.chromium.org//2369953004 .
diff --git a/pubspec.yaml b/pubspec.yaml
index 9ea1832..3981abb 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: async
-version: 1.11.1
+version: 1.11.2
 author: Dart Team <misc@dartlang.org>
 description: Utility functions and classes related to the 'dart:async' library.
 homepage: https://www.github.com/dart-lang/async
diff --git a/test/stream_group_test.dart b/test/stream_group_test.dart
index 5078dad..76b69b9 100644
--- a/test/stream_group_test.dart
+++ b/test/stream_group_test.dart
@@ -507,7 +507,7 @@
         await flushMicrotasks();
         controller.add("second");
 
-        expect(streamGroup.remove(controller.stream), isNull);
+        expect(streamGroup.remove(controller.stream), completion(null));
         expect(streamGroup.close(), completes);
       });
 
@@ -548,7 +548,7 @@
         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);