pkg/async: cleanup unused variables in tests

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 727077cebd72d76b1a71168bfaa876acf35b799f..db6c9387136cc4f39c1a94710baf9d80133c376d 100644
--- a/test/stream_group_test.dart
+++ b/test/stream_group_test.dart
@@ -472,9 +472,8 @@ void regardlessOfType(StreamGroup<String> newStreamGroup()) {
     });
 
     group("while active", () {
-      var subscription;
       setUp(() async {
-        subscription = streamGroup.stream.listen(null);
+        streamGroup.stream.listen(null);
         await flushMicrotasks();
       });