Remove package:unittest from some tests

tests/lib/async/stream_periodic5_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';
 
 watchMs(Stopwatch watch) {
   int microsecs = watch.elapsedMicroseconds;
   // Give it some slack. The Stopwatch is more precise than the timers. This
   // means that we sometimes get 3995 microseconds instead of 4+ milliseconds.
   // 200 microseconds should largely account for this discrepancy.
   return (microsecs + 200) ~/ 1000;
 }
 
 main() {
@@ skipping 11 matching lines @@
         while (watch2.elapsedMilliseconds < 15) { }
         // Make sure the stream can be paused when it has overdue events.
         // We just busy waited for 15ms, even though the stream is supposed to
         // emit events every 5ms.
         subscription.pause();
         watch.stop();
         new Timer(const Duration(milliseconds: 150), () {
           watch.start();
           subscription.resume();
         });
-      } 
+      }
     }, onDone: expectAsync(() { }));
   });
 }