Add conversion functions for our own tree format.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>

library/HTMLPurifier/Token/End.php

@@ -15,6 +15,10 @@ class HTMLPurifier_Token_End extends HTMLPurifier_Token_Tag
      * @type HTMLPurifier_Token
      */
     public $start;
+
+    public function toNode() {
+        throw new Exception("HTMLPurifier_Token_End->toNode not supported!");
+    }
 }
 
 // vim: et sw=4 sts=4