Use metadata annotation @patch for patch classes

runtime/bin/file_system_entity_patch.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.
 
-patch class FileStat {
-  /* patch */ static _statSync(String path) native "File_Stat";
+@patch class FileStat {
+  /* @patch */ static _statSync(String path) native "File_Stat";
 }
 
 
-patch class FileSystemEntity {
-  /* patch */ static _getType(String path, bool followLinks)
+@patch class FileSystemEntity {
+  /* @patch */ static _getType(String path, bool followLinks)
       native "File_GetType";
-  /* patch */ static _identical(String path1, String path2)
+  /* @patch */ static _identical(String path1, String path2)
       native "File_AreIdentical";
-  /* patch */ static _resolveSymbolicLinks(String path)
+  /* @patch */ static _resolveSymbolicLinks(String path)
       native "File_ResolveSymbolicLinks";
 }