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

test/stream_group_test.dart

Index: test/stream_group_test.dart
diff --git a/test/stream_group_test.dart b/test/stream_group_test.dart
index 5078dad474e3fef9da9c61b13f2cf0fdcd62ef79..76b69b95e1f59136756063b134624677af783665 100644
--- a/test/stream_group_test.dart
+++ b/test/stream_group_test.dart
@@ -507,7 +507,7 @@ void regardlessOfType(StreamGroup<String> newStreamGroup()) {
         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 @@ void regardlessOfType(StreamGroup<String> newStreamGroup()) {
         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);