pkg/async: cleanup unused variables in tests

test/stream_zip_zone_test.dart

 // Copyright (c) 2013, 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:test/test.dart";
 
 // Test that stream listener callbacks all happen in the zone where the
 // listen occurred.
 
@@ skipping 23 matching lines @@
       Zone newZone1 = Zone.current;
       StreamSubscription sub;
       sub = stream.listen(expectAsync((v) {
         expect(v, 42);
         expect(Zone.current, newZone1);
         outer.run(() {
           sub.onData(expectAsync((v) {
             expect(v, 37);
             expect(Zone.current, newZone1);
             runZoned(() {
-              Zone newZone2 = Zone.current;
               sub.onData(expectAsync((v) {
                 expect(v, 87);
                 expect(Zone.current, newZone1);
               }));
             });
             if (controller is SynchronousStreamController) {
               scheduleMicrotask(() => controller.add(87));
             } else {
               controller.add(87);
             }
           }));
         });
         if (controller is SynchronousStreamController) {
           scheduleMicrotask(() => controller.add(37));
         } else {
           controller.add(37);
         }
       }));
     });
     controller.add(42);
   });
 }