Remove package:unittest from some tests

tests/lib/async/stream_join_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 basic StreamController and StreamController.singleSubscription.
 library stream_join_test;
 
 import 'dart:async';
 import 'event_helper.dart';
-import 'package:unittest/unittest.dart';
-import "package:expect/expect.dart";
+import 'package:test/test.dart';
 
 main() {
   test("join-empty", () {
     StreamController c = new StreamController();
     c.stream.join("X").then(expectAsync(
       (String s) => expect(s, equals(""))
     ));
     c.close();
   });
 
@@ skipping 44 matching lines @@
 class Foo {
   String value;
   Foo(this.value);
   String toString() => value;
 }
 
 class Bad {
   Bad();
   String toString() => throw "BAD!";
 }