Remove package:unittest from some tests

tests/lib/async/stream_periodic6_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';
 
 main() {
   test("stream-periodic1", () {
     Stream stream = new Stream.periodic(const Duration(milliseconds: 1),
                                         (i) {
                                           if (i == 3) throw 42;
                                           return i;
                                         });
     int expected = 0;
     var subscription;
     subscription = stream.listen(expectAsync((data) {
       expect(data, expected++);
       if (expected == 5) subscription.cancel();
     }, count: 4),
     onError: expectAsync((e, s) {
       expect(e, 42);
       expected++;
     }));
   });
 }