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

sdk/lib/io/link.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.
 
 part of dart.io;
 
 /**
  * [Link] objects are references to filesystem links.
  *
  */
-abstract class Link implements FileSystemEntity {
+abstract class Link extends FileSystemEntity {
   /**
    * Creates a Link object.
    */
   factory Link(String path) => new _Link(path);
 
   /**
    * Create a Lint object from a URI.
    *
    * If [uri] cannot reference a link this throws [UnsupportedError].
    */
@@ skipping 270 matching lines @@
         return new ArgumentError();
       case _OSERROR_RESPONSE:
         var err = new OSError(response[_OSERROR_RESPONSE_MESSAGE],
                               response[_OSERROR_RESPONSE_ERROR_CODE]);
         return new FileSystemException(message, path, err);
       default:
         return new Exception("Unknown error");
     }
   }
 }