Remove package:unittest from some tests

tests/lib/async/stream_periodic4_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';
 
 void runTest(period, maxElapsed, pauseDuration) {
   Function done = expectAsync(() { });
 
   Stopwatch watch = new Stopwatch()..start();
   Stream stream = new Stream.periodic(period, (x) => x);
   var subscription;
   subscription = stream.take(5).listen((i) {
     if (watch.elapsed > maxElapsed) {
       // Test failed in this configuration. Try with more time (or give up
@@ skipping 21 matching lines @@
   }, onDone: done);
 }
 
 main() {
   test("stream-periodic4", () {
     runTest(const Duration(milliseconds: 2),
             const Duration(milliseconds: 8),
             const Duration(milliseconds: 10));
   });
 }