Remove package:unittest from some tests

tests/lib/async/stream_state_nonzero_timer_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 the event/callback protocol of the stream implementations.
 // Uses a non-zero timer so it fails on d8.
 
 library stream_state_nonzero_timer_test;
 
 import "dart:async";
-import "package:unittest/unittest.dart";
+import "package:test/test.dart";
 import "stream_state_helper.dart";
 
 const ms5 = const Duration(milliseconds: 5);
 
 // Testing pause/resume, some with non-zero duration. This only makes sense for
 // non-broadcast streams, since broadcast stream subscriptions handle their
 // own pauses.
 
 main() {
   var p = "StreamController";
@@ skipping 71 matching lines @@
        t.pause();
        t.resume();
        t.close();
      })
      ..expectCancel()
      ..expectDone(t.terminate);
     t..listen()
      ..add(42);
   });
 }