Make _FileSystemWatcher.watch private.

sdk/lib/_internal/compiler/js_lib/io_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.
 
 import 'dart:_js_helper' show patch;
 
 @patch
 class _Directory {
   @patch
   static _current() {
@@ skipping 494 matching lines @@
   }
   @patch
   int _terminalLines(int fd) {
     throw new UnsupportedError("Stdout.terminalLines");
   }
 }
 
 @patch
 class _FileSystemWatcher {
   @patch
-  static Stream<FileSystemEvent> watch(
+  static Stream<FileSystemEvent> _watch(
       String path, int events, bool recursive) {
     throw new UnsupportedError("_FileSystemWatcher.watch");
   }
   @patch
   static bool get isSupported {
     throw new UnsupportedError("_FileSystemWatcher.isSupported");
   }
 }
 
 @patch
 class _IOService {
   @patch
   static Future _dispatch(int request, List data) {
     throw new UnsupportedError("_IOService._dispatch");
   }
 }