[dart:io] Update docs for FileSystemEvent.isDirectory for Windows

sdk/lib/io/file_system_entity.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.
 
 part of dart.io;
 
 /**
  * The type of an entity on the file system, such as a file, directory, or link.
  *
  * These constants are used by the [FileSystemEntity] class
@@ skipping 738 matching lines @@
   final int type;
 
   /**
    * The path that triggered the event. Depending on the platform and the
    * FileSystemEntity, the path may be relative.
    */
   final String path;
 
   /**
    * Is `true` if the event target was a directory.
+   *
+   * Note that if the file has been deleted by the time the event has arrived,
+   * this will always be `false` on Windows. In particular, it will always be
+   * `false` for `DELETE` events.
    */
   final bool isDirectory;
 
   FileSystemEvent._(this.type, this.path, this.isDirectory);
 }
 
 /**
  * File system event for newly created file system objects.
  */
 class FileSystemCreateEvent extends FileSystemEvent {
@@ skipping 50 matching lines @@
     buffer.write(')');
     return buffer.toString();
   }
 }
 
 class _FileSystemWatcher {
   external static Stream<FileSystemEvent> _watch(
       String path, int events, bool recursive);
   external static bool get isSupported;
 }