Migrated test block 168 to Dart 2.0.

tests/lib_2/async/future_constructor_test.dart

 // Copyright (c) 2012, 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_test;
 
 import "package:expect/expect.dart";
 import 'dart:async';
 
 main() {
@@ skipping 23 matching lines @@
     throw "Flif";
   });
   compare(() => err1);
   bool hasExecuted = false;
   compare(() {
     hasExecuted = true;
     return 499;
   });
   Expect.isFalse(hasExecuted);
 }