Refactor Future constructors.

utils/pub/validator/pubspec_field.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 '../entrypoint.dart';
 import '../system_cache.dart';
@@ skipping 38 matching lines @@
     var description = pubspec.fields['description'];
     if (description == null) {
       errors.add('Your pubspec.yaml is missing a "description" field.');
     }
 
     var version = pubspec.fields['version'];
     if (version == null) {
       errors.add('Your pubspec.yaml is missing a "version" field.');
     }
 
-    return new Future.immediate(null);
+    return new Future.value();
   }
 }