Migrated test block 168 to Dart 2.0.

tests/lib_2/async/future_value_chain4_test.dart

Index: tests/lib_2/async/future_value_chain4_test.dart
diff --git a/tests/lib_strong/async/future_value_chain4_test.dart b/tests/lib_2/async/future_value_chain4_test.dart
similarity index 85%
rename from tests/lib_strong/async/future_value_chain4_test.dart
rename to tests/lib_2/async/future_value_chain4_test.dart
index 38952e9c33ab2c218073bc1cbaef467b9f7991ae..ea0a887fa82dffc773136586bb1d2832e8f62203 100644
--- a/tests/lib_strong/async/future_value_chain4_test.dart
+++ b/tests/lib_2/async/future_value_chain4_test.dart
@@ -6,11 +6,12 @@ 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;