Migrate block 163 to Dart 2.0.

tests/language_strong/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
-  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());
-}