Refactor Future constructors.

utils/pub/validator/directory.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 directory_validator;
 
 import 'dart:async';
 
 import 'package:pathos/path.dart' as path;
 
 import '../entrypoint.dart';
 import '../io.dart';
 import '../utils.dart';
 import '../validator.dart';
 
 /// A validator that validates a package's top-level directories.
 class DirectoryValidator extends Validator {
   DirectoryValidator(Entrypoint entrypoint)
     : super(entrypoint);
 
   static final _PLURAL_NAMES = ["tools", "tests", "docs", "examples"];
 
   Future validate() {
-    return new Future.of(() {
+    return new Future.sync(() {
       for (var dir in listDir(entrypoint.root.dir)) {
         if (!dirExists(dir)) continue;
 
         dir = path.basename(dir);
         if (_PLURAL_NAMES.contains(dir)) {
           // Cut off the "s"
           var singularName = dir.substring(0, dir.length - 1);
           warnings.add('Rename the top-level "$dir" directory to '
                   '"$singularName".\n'
               'The Pub layout convention is to use singular directory '
                   'names.\n'
               'Plural names won\'t be correctly identified by Pub and other '
                   'tools.');
         }
 
         if (dir.contains(new RegExp(r"^samples?$"))) {
           warnings.add('Rename the top-level "$dir" directory to "example".\n'
               'This allows Pub to find your examples and create "packages" '
                   'directories for them.\n');
         }
       }
     });
   }
 }