Migrated test block 168 to Dart 2.0.

tests/lib_2/async/future_constructor2_test.dart

 // Copyright (c) 2015, 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.
 
 library future_delayed_test;
 
 import 'package:async_helper/async_helper.dart';
 import "package:expect/expect.dart";
 import 'dart:async';
 
@@ skipping 39 matching lines @@
   // Test that all the Future constructors take functions that return a Future
   // as argument.
   // In particular the constructors must not type their argument as
   // `T computation()`.
   unnamed();
   delayed();
   microtask();
   sync();
   asyncEnd();
 }