Migrated test block 168 to Dart 2.0.

tests/lib/async/future_value_chain_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);
-  errorFuture.catchError((x) {
-    Expect.equals(499, x);
-    var valueChainFuture = new Future.value(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();
-    });
-  });
-}