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

sdk/lib/io/file.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;
 
 /**
  * FileMode describes the modes in which a file can be opened.
  */
 class FileMode {
@@ skipping 27 matching lines @@
  * link, except for File.delete and File.deleteSync, which operate on
  * the link.
  *
  * To operate on the underlying file data there are two options:
  *
  *  * Use streaming: read the contents of the file from the [Stream]
  *    this.[openRead]() and write to the file by writing to the [IOSink]
  *    this.[openWrite]().
  *  * Open the file for random access operations using [open].
  */
-abstract class File implements FileSystemEntity {
+abstract class File extends FileSystemEntity {
   /**
    * Create a File object.
    */
   factory File(String path) => new _File(path);
 
   /**
    * Create a File object from a URI.
    *
    * If [uri] cannot reference a file this throws [UnsupportedError].
    */
@@ skipping 547 matching lines @@
       sb.write(": $osError");
       if (path != null) {
         sb.write(", path = '$path'");
       }
     } else if (path != null) {
       sb.write(": $path");
     }
     return sb.toString();
   }
 }