Migrated test block 168 to Dart 2.0.

tests/lib_2/async/future_value_chain2_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';
 
 main() {
   asyncStart();
   var errorFuture = new Future.error(499);
   var valueChainFuture = new Future.sync(() => errorFuture);
   // The errorFuture must not be propagated immediately as we would otherwise
   // not have time to catch the error.
   valueChainFuture.catchError((error) {
     Expect.equals(499, error);
     asyncEnd();
   });
 }