Add type checks to directory listing.

runtime/bin/directory.dart

 // Copyright (c) 2011, 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.
 
 interface Directory factory _Directory {
   /**
    * Creates a directory object. The path is either a full path or
    * relative to the directory in which the Dart VM was
    * started.
    */
@@ skipping 41 matching lines @@
   void setFileHandler(void fileHandler(String file));
 
   /**
    * Set the done handler that is called when a directory listing is
    * done. The handler is called with an indication of whether or not
    * the listing operation completed.
    */
   void setDoneHandler(void doneHandler(bool completed));
 
   /**
-   * Sets the error handler that is called on errors listing
+   * Sets the error handler that is called on error listing
    * directories.
    */
   void setErrorHandler(void errorHandler(String error));
 
   /**
    * Gets the path of this directory.
    */
   final String path;
 }
+
+
+class DirectoryException {
+  const DirectoryException(String this.message);
+  String toString() => "DirectoryException: $message";
+  final String message;
+}