Remove package:unittest from some tests

tests/lib/async/stream_periodic3_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';
 
 // The stopwatch is more precise than the Timer.
 // Some browsers (Firefox and IE so far) can trigger too early. So we add more
 // margin. We use identical(1, 1.0) as an easy way to know if the test is
 // compiled by dart2js.
 int get safetyMargin => identical(1, 1.0) ? 5 : 0;
 
 main() {
   test("stream-periodic3", () {
     Stopwatch watch = new Stopwatch()..start();
     Stream stream = new Stream.periodic(const Duration(milliseconds: 1),
                                         (x) => x);
     stream.take(10).listen((_) { }, onDone: expectAsync(() {
       int millis = watch.elapsedMilliseconds + safetyMargin;
       expect(millis, greaterThanOrEqualTo(10));
     }));
   });
 }