Make File, Link and Directory extend, not implement, FileSystemEntity.

sdk/lib/io/directory.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.
 
 part of dart.io;
 
 /**
  * A reference to a directory (or _folder_) on the file system.
  */
-abstract class Directory implements FileSystemEntity {
+abstract class Directory extends FileSystemEntity {
   /**
    * Gets the path of this directory.
    */
   final String path;
 
   /**
    * Creates a directory object. The path is either an absolute path,
    * or it is a relative path which is interpreted relative to the directory
    * in which the Dart VM was started.
    */
@@ skipping 166 matching lines @@
    * directories, files, and links.
    */
   List<FileSystemEntity> listSync({bool recursive: false,
                                    bool followLinks: true});
 
   /**
    * Returns a human readable string for this Directory instance.
    */
   String toString();
 }