Remove package:unittest from some tests

tests/lib/async/stream_iterator_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.
 
 import "dart:async";
-import "package:unittest/unittest.dart";
+import "package:test/test.dart";
 
 main() {
   test("stream iterator basic", () async {
     var stream = createStream();
     StreamIterator iterator = new StreamIterator(stream);
     expect(iterator.current, isNull);
     expect(await iterator.moveNext(), isTrue);
     expect(iterator.current, 42);
     expect(await iterator.moveNext(), isTrue);
     expect(iterator.current, 37);
@@ skipping 69 matching lines @@
 
 /// Create a stream that throws when cancelled.
 Stream createCancelErrorStream() async* {
   int i = 0;
   try {
     while (true) yield i++;
   } finally {
     throw "BAD";
   }
 }