Refactor Future constructors.

tests/standalone/io/file_system_delete_test.dart

 // Copyright (c) 2013, 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.
 
 import "package:expect/expect.dart";
 import "dart:async";
 import "dart:io";
 
 Future throws(callback()) {
-  return new Future.immediate(null)
+  return new Future.value()
     .then((_) => callback())
     .then((_) { throw "Expected error"; },
           onError: (_) {});
 }
 
 void testDeleteFileSync() {
   var tmp = new Directory("").createTempSync();
   var path = "${tmp.path}${Platform.pathSeparator}";
 
   var file = new File("${path}myFile");
@@ skipping 377 matching lines @@
   testDeleteDirectory();
   if (Platform.operatingSystem != 'windows') {
     testDeleteFileLinkSync();
     testDeleteFileLink();
   }
   testDeleteDirectoryLinkSync();
   testDeleteDirectoryLink();
   testDeleteBrokenLinkSync();
   testDeleteBrokenLink();
 }