Migrated test block 168 to Dart 2.0.

tests/lib_2/async/catch_errors_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() {
   asyncStart();
 
   // Make sure `catchErrors` does not execute the error callback.
   catchErrors(() {
     return 'allDone';
   }).listen((x) {
     Expect.fail("Unexpected callback");
   });
 
   // Wait one cycle before shutting down the test.
   Timer.run(asyncEnd);
 }