Refactor Future constructors.

utils/pub/validator/license.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 pubspec_field_validator;
 
 import 'dart:async';
 
 import 'package:pathos/path.dart' as path;
 
 import '../entrypoint.dart';
 import '../io.dart';
 import '../system_cache.dart';
 import '../validator.dart';
 
 /// A validator that checks that a LICENSE-like file exists.
 class LicenseValidator extends Validator {
   LicenseValidator(Entrypoint entrypoint)
     : super(entrypoint);
 
   Future validate() {
-    return new Future.of(() {
+    return new Future.sync(() {
       var licenseLike = new RegExp(
           r"^([a-zA-Z0-9]+[-_])?(LICENSE|COPYING)(\..*)?$");
       if (listDir(entrypoint.root.dir)
           .map(path.basename)
           .any(licenseLike.hasMatch)) {
         return;
       }
 
       errors.add(
           "You must have a COPYING or LICENSE file in the root directory.\n"
           "An open-source license helps ensure people can legally use your "
           "code.");
     });
   }
 }