Clean up:

lib/src/util.dart

-library markdown.util;
+library markdown.src.util;
 
 /// Replaces `<`, `&`, and `>`, with their HTML entity equivalents.
 String escapeHtml(String html) {
-  if (html == '' || html == null) return null;
   return html
       .replaceAll('&', '&amp;')
       .replaceAll('<', '&lt;')
       .replaceAll('>', '&gt;');
 }
-
-/// Removes null or empty values from [map].
-void cleanMap(Map map) {
-  map.keys.where((e) => isNullOrEmpty(map[e])).toList().forEach(map.remove);
-}
-
-/// Returns true if an object is null or an empty string.
-bool isNullOrEmpty(object) {
-  return object == null || object == '';
-}