Consistently use implements when one interface inherits from another interface

pkg/analyzer/lib/file_system/file_system.dart

 // Copyright (c) 2014, 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.
 
 library file_system;
 
 import 'dart:async';
 
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/util/absolute_path.dart';
 import 'package:path/path.dart';
 import 'package:watcher/watcher.dart';
 
 /**
  * [File]s are leaf [Resource]s which contain data.
  */
-abstract class File extends Resource {
+abstract class File implements Resource {
   /**
    * Watch for changes to this file
    */
   Stream<WatchEvent> get changes;
 
   /**
    * Return the last-modified stamp of the file.
    * Throws [FileSystemException] if the file does not exist.
    */
   int get modificationStamp;
@@ skipping 18 matching lines @@
   final String message;
 
   FileSystemException(this.path, this.message);
 
   String toString() => 'FileSystemException(path=$path; message=$message)';
 }
 
 /**
  * [Folder]s are [Resource]s which may contain files and/or other folders.
  */
-abstract class Folder extends Resource {
+abstract class Folder implements Resource {
   /**
    * Watch for changes to the files inside this folder (and in any nested
    * folders, including folders reachable via links).
    */
   Stream<WatchEvent> get changes;
 
   /**
    * If the path [path] is a relative path, convert it to an absolute path
    * by interpreting it relative to this folder.  If it is already an aboslute
    * path, then don't change it.
@@ skipping 134 matching lines @@
 
   @override
   Uri restoreAbsolute(Source source) =>
       _provider.pathContext.toUri(source.fullName);
 
   /**
    * Return `true` if the given [uri] is a `file` URI.
    */
   static bool _isFileUri(Uri uri) => uri.scheme == _FILE_SCHEME;
 }