Remove package:unittest from some tests

tests/lib/async/stream_subscription_cancel_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.
 
 // Test the StreamSubscription.cancel return Future.
 library stream_subscription_cancel;
 
 import 'dart:async';
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 
 void main() {
   test('subscription.cancel', () {
     var completer = new Completer();
     StreamController controller = new StreamController(
         onCancel: () => completer.future);
 
     bool done = false;
     expect(controller.stream.listen(null).cancel().then((_) => done),
            completion(equals(true)));
@@ skipping 293 matching lines @@
     var future = controller.stream.where((x) => true).listen(null).cancel();
 
     expect(future.then((_) => done = true), completion(equals(true)));
 
     Timer.run(() {
       expect(done, isFalse);
       completer.complete();
     });
   });
 }