Remove package:unittest from some tests

tests/lib/async/stream_state_helper.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.
 
 library stream_state_helper;
 
-import "package:unittest/unittest.dart";
+import "package:test/test.dart";
 import "dart:async";
 import "dart:collection";
 
 class SubscriptionProtocolTest {
   final StreamProtocolTest _streamTest;
   final int id;
   StreamSubscription _subscription;
 
   SubscriptionProtocolTest(this.id, this._subscription, this._streamTest);
 
@@ skipping 562 matching lines @@
   }
 
   bool _testBroadcastCancel() {
     _actual = "*[BroadcastCancel]";
     return true;
   }
 
   /** Returns a representation of the event it was tested against. */
   String toString() => _actual;
 }