Migrate block 163 to Dart 2.0.

tests/language_2/unused_overridden_async_test.dart

 // Copyright (c) 2016, 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 'dart:async';
 
 class Base {
   Future<int> method() async {
     throw 'Should be unused';
   }
 }
 
 class Sub1 extends Base {
   @override

[Lasse Reichstein Nielsen, 2017/08/29 08:49:44]

Remove @override. We don't use that in the platform libraries, so we shouldn't
be using it in the tests either.

[jcollins, 2017/08/29 17:39:11]

Done.

   Future<int> method() async {
     return 1;
   }
 }
 
 class Sub2 extends Base {
   @override
   Future<int> method() async {
     return 2;
   }
 }
 
 help(Base object) async {
   print(await object.method());
 }
 
 main() async {
   await help(new Sub1());
   await help(new Sub2());
 }