Remove package:unittest from some tests

tests/lib/async/stream_periodic2_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 merging streams.
 library dart.test.stream_from_iterable;
 
 import "dart:async";
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 
 main() {
   test("stream-periodic2", () {
     Stream stream = new Stream.periodic(const Duration(milliseconds: 1),
                                         (x) => x);
     int receivedCount = 0;
     var subscription;
     subscription = stream.listen(expectAsync((data) {
       expect(data, receivedCount);
       receivedCount++;
       if (receivedCount == 5) subscription.cancel();
     }, count: 5));
   });
 }