Refactor Future constructors.

utils/pub/validator/dependency.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 dependency_validator;
 
 import 'dart:async';
 
 import '../entrypoint.dart';
 import '../hosted_source.dart';
@@ skipping 13 matching lines @@
     return Future.forEach(entrypoint.root.pubspec.dependencies, (dependency) {
       if (dependency.source is! HostedSource) {
         return _warnAboutSource(dependency);
       }
 
       if (dependency.name == entrypoint.root.name) {
         warnings.add('You don\'t need to explicitly depend on your own '
                 'package.\n'
             'Pub enables "package:${entrypoint.root.name}" imports '
                 'implicitly.');
-        return new Future.immediate(null);
+        return new Future.value();
       }
 
       if (dependency.constraint.isAny) _warnAboutConstraint(dependency);
 
-      return new Future.immediate(null);
+      return new Future.value();
     });
   }
 
   /// Warn that dependencies should use the hosted source.
   Future _warnAboutSource(PackageRef ref) {
     return entrypoint.cache.sources['hosted']
         .getVersions(ref.name, ref.name)
         .catchError((e) => <Version>[])
         .then((versions) {
       var constraint;
@@ skipping 42 matching lines @@
   }
 
   /// Returns the suggested version constraint for a dependency that was tested
   /// against [version].
   String _constraintForVersion(Version version) {
     if (version.major != 0) return '">=$version <${version.major + 1}.0.0"';
     return '">=$version <${version.major}.${version.minor}.'
         '${version.patch + 1}"';
   }
 }