Fix move-watched-dir for MacOS.

runtime/bin/file_patch.dart

 // 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.
 
 patch class _File {
   /* patch */ static _exists(String path) native "File_Exists";
   /* patch */ static _create(String path) native "File_Create";
   /* patch */ static _createLink(String path, String target)
       native "File_CreateLink";
   /* patch */ static _linkTarget(String path) native "File_LinkTarget";
@@ skipping 65 matching lines @@
       return;
     }
     var socket = new _RawSocket(new _NativeSocket.watch(socketId));
     _subscription = socket.expand((event) {
       bool stop = false;
       var events = [];
       var pair = {};
       if (event == RawSocketEvent.READ) {
         String getPath(event) {
           var path = _path;
-          if (event[2] != null) {
+          if (event[2] != null && event[2].isNotEmpty) {
             path += Platform.pathSeparator;
             path += event[2];
           }
           return path;
         }
         bool getIsDir(event) {
           if (Platform.isWindows) {
             // Windows does not get 'isDir' as part of the event.
             return FileSystemEntity.isDirectorySync(getPath(event));
           }
@@ skipping 76 matching lines @@
 
   int _watchPath(String path, int events, bool recursive)
       native "FileSystemWatcher_WatchPath";
   void _unwatchPath() native "FileSystemWatcher_UnwatchPath";
   List _readEvents() native "FileSystemWatcher_ReadEvents";
 }
 
 Uint8List _makeUint8ListView(Uint8List source, int offsetInBytes, int length) {
   return new Uint8List.view(source.buffer, offsetInBytes, length);
 }