Refactor Future constructors.

utils/pub/validator/lib.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 lib_validator;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:pathos/path.dart' as path;
 
 import '../entrypoint.dart';
 import '../io.dart';
 import '../system_cache.dart';
 import '../utils.dart';
 import '../validator.dart';
 
 // TODO(nweiz): When issue 7196 is fixed, complain about non-Dart files in lib.
 /// A validator that checks that libraries in "lib/" (and not "lib/src/") exist
 /// and are well-formed.
 class LibValidator extends Validator {
   LibValidator(Entrypoint entrypoint)
     : super(entrypoint);
 
   Future validate() {
-    return new Future.of(() {
+    return new Future.sync(() {
       var libDir = path.join(entrypoint.root.dir, "lib");
 
       if (!dirExists(libDir)) {
         errors.add('You must have a "lib" directory.\n'
             "Without that, users cannot import any code from your package.");
         return;
       }
 
       var files = listDir(libDir)
           .map((file) => path.relative(file, from: libDir))
           .toList();
       if (files.isEmpty) {
         errors.add('You must have a non-empty "lib" directory.\n'
             "Without that, users cannot import any code from your package.");
       } else if (files.length == 1 && files.first == "src") {
         errors.add('The "lib" directory must contain something other than '
             '"src".\n'
             "Otherwise, users cannot import any code from your package.");
       }
     });
   }
 }