Get rid of unneeded Future.immediate() calls.

utils/pub/system_cache.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 system_cache;
 
 import 'dart:io';
 import 'dart:async';
 
 import 'git_source.dart';
@@ skipping 75 matching lines @@
   Future<Directory> createTempDir() {
     return ensureDir(tempDir).then((temp) {
       return io.createTempDir(join(temp, 'dir'));
     });
   }
 
   /// Delete's the system cache's internal temp directory.
   Future deleteTempDir() {
     log.fine('Clean up system cache temp directory $tempDir.');
     return dirExists(tempDir).then((exists) {
-      if (!exists) return new Future.immediate(null);
+      if (!exists) return;
       return deleteDir(tempDir);
     });
   }
 }