Remove package:unittest from some tests

tests/language/async_star_pause_test.dart

 // Copyright (c) 2015, 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 async_star_pause_test;
 
-import "package:unittest/unittest.dart";
+import "package:test/test.dart";
 import "dart:async";
 
 main() {
   test("await for pauses stream during body", () async {
     // Assumes await-for uses streamIterator.
     var log = [];
     var s = () async* {
       for (int i = 0; i < 3; i++) {
         log.add("$i-");
         yield i;
         // Should pause here until next iteration of await-for loop.
         log.add("$i+");
       }
     }();
     await for (var i in s) {
       log.add("$i?");
       await nextMicrotask();
       log.add("$i!");
     }
     expect(log, ["0-", "0?", "0!", "0+",
                  "1-", "1?", "1!", "1+",
                  "2-", "2?", "2!", "2+"]);
   });
 }
 Future nextMicrotask() => new Future.microtask((){});