pkg/async: cleanup unused variables in tests

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.
 
 library async.test.stream_group_test;
 
 import 'dart:async';
 
 import 'package:async/async.dart';
 import 'package:test/test.dart';
@@ skipping 454 matching lines @@
         streamGroup.add(controller.stream);
         streamGroup.add(controller.stream);
 
         // If the stream was actually listened to multiple times, this would
         // throw a StateError.
         streamGroup.stream.listen(null);
       });
     });
 
     group("while active", () {
-      var subscription;
       setUp(() async {
-        subscription = streamGroup.stream.listen(null);
+        streamGroup.stream.listen(null);
         await flushMicrotasks();
       });
 
       test("listens to the stream immediately", () async {
         var controller = new StreamController<String>();
 
         expect(streamGroup.add(controller.stream), isNull);
         await flushMicrotasks();
         expect(controller.hasListener, isTrue);
       });
@@ skipping 227 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);