tests: import unittest via package: url

tests/lib/async/stream_periodic_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 '../../../pkg/unittest/lib/unittest.dart';
+import 'package:unittest/unittest.dart';
 
 main() {
   test("stream-periodic1", () {
     Stream stream = new Stream.periodic(const Duration(milliseconds: 1));
     int receivedCount = 0;
     var subscription;
     subscription = stream.listen(expectAsync((data) {
       expect(data, isNull);
       receivedCount++;
       if (receivedCount == 5) subscription.cancel();
     }, count: 5));
   });
 }