dart:io | Add asynchronous versions of the methods of FileSystemEntity and Link.

tests/standalone/io/windows_file_system_links_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:io";
 import "dart:isolate";
 
 testJunctionTypeDelete() {
   var temp = new Directory('').createTempSync();
@@ skipping 63 matching lines @@
                     FileSystemEntity.typeSync(x, followLinks: false));
       Expect.throws(() => new Link(y).targetSync());
 
       temp.deleteSync(recursive: true);
     });
   });
 }
 
 
 main() {
+  // Links on other platforms are tested by file_system_[async_]links_test.
   if (Platform.operatingSystem == 'windows') {
     testJunctionTypeDelete();
   }
 }