Add FileSystemWatcher class to dart:io.

runtime/bin/builtin_impl_sources.gypi

 # Copyright (c) 2012, 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.
 
 # This file contains all C++ sources for the dart:builtin library and
 # some of the C++ sources for the dart:io library.  The rest are in
 # io_impl_sources.gypi.
 {
   'sources': [
     'io_buffer.cc',
@@ skipping 23 matching lines @@
     'extensions_linux.cc',
     'extensions_macos.cc',
     'extensions_win.cc',
     'file.cc',
     'file.h',
     'file_android.cc',
     'file_linux.cc',
     'file_macos.cc',
     'file_win.cc',
     'file_test.cc',
+    'file_system_watcher.cc',
+    'file_system_watcher.h',
+    'file_system_watcher_android.cc',
+    'file_system_watcher_linux.cc',
+    'file_system_watcher_macos.cc',
+    'file_system_watcher_win.cc',
     'fdutils.h',
     'fdutils_android.cc',
     'fdutils_linux.cc',
     'fdutils_macos.cc',
     'hashmap_test.cc',
     'isolate_data.h',
     'native_service.h',
     'native_service.cc',
     'thread.h',
     'utils.h',
     'utils_android.cc',
     'utils_linux.cc',
     'utils_macos.cc',
     'utils_win.cc',
   ],
 }