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

sdk/lib/io/file_system_entity.dart

Index: sdk/lib/io/file_system_entity.dart
diff --git a/sdk/lib/io/file_system_entity.dart b/sdk/lib/io/file_system_entity.dart
index 2641526427edb9a71ad837e9f5e5d285964c2852..9281b5941cc11368d3845a17b662b4a7e11fb431 100644
--- a/sdk/lib/io/file_system_entity.dart
+++ b/sdk/lib/io/file_system_entity.dart
@@ -756,6 +756,10 @@ class FileSystemEvent {
 
   /**
    * 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;