Consistently use implements when one interface inherits from another interface

pkg/analyzer/lib/file_system/file_system.dart

Index: pkg/analyzer/lib/file_system/file_system.dart
diff --git a/pkg/analyzer/lib/file_system/file_system.dart b/pkg/analyzer/lib/file_system/file_system.dart
index 21e8d05112df3b3aed1625c5e140ca2077e13431..cbe175f89a27387dba986d59e23f6d3d37efefef 100644
--- a/pkg/analyzer/lib/file_system/file_system.dart
+++ b/pkg/analyzer/lib/file_system/file_system.dart
@@ -14,7 +14,7 @@ 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
    */
@@ -53,7 +53,7 @@ class FileSystemException implements Exception {
 /**
  * [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).