Migrated test block 171 to Dart 2.0.

tests/lib_2/async/run_zoned8_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 'package:async_helper/async_helper.dart';
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'catch_errors.dart';
 
 main() {
@@ skipping 22 matching lines @@
       "main exit",
       1,
       "error: 1",
       2,
       "error: 2",
     ], events);
     asyncEnd();
   });
   events.add("main exit");
 }