Remove package:unittest from some tests

tests/lib/async/first_regression_test.dart

 // Copyright (c) 2012, 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.
 
 // Regression test for http://dartbug.com/7964
 
 library first_regression_test;
 import 'dart:async';
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 
 class DoubleTransformer<T> implements StreamTransformer<T, T> {
   Stream<T> bind(Stream<T> stream) {
     var transformer = new StreamTransformer<T, T>.fromHandlers(
         handleData: (T data, EventSink<T> sink) {
           sink.add(data);
           sink.add(data);
         });
     return transformer.bind(stream);
   }
 }
 
 main() {
   test("Double event before first", () {
     // This should not crash. Did crash by trying to complete future more
     // than once.
     new Stream.fromIterable([1, 2])
         .transform(new DoubleTransformer())
         .first
         .then(expectAsync((e) {}));
   });
 }