Migrated test block 168 to Dart 2.0.

tests/lib_2/async/future_value_chain4_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';
 
-class MyFuture implements Future {
-  then(valueHandler, {onError}) {
+class MyFuture<T> implements Future<T> {
+  Future<S> then<S>(FutureOr<S> valueHandler(T), {Function onError}) {
     scheduleMicrotask(() {
       valueHandler(499);
     });
+    return null;
   }
 
   catchError(_, {test}) => null;
   whenComplete(_) => null;
   asStream() => null;
   timeout(Duration timeLimit, {void onTimeout()}) => null;
 }
 
 main() {
   asyncStart();
   Completer completer = new Completer();
   completer.complete(new MyFuture());
   Expect.isTrue(completer.isCompleted);
   Expect.throws(() => completer.complete(42));
   completer.future.then((_) => asyncEnd());
 }