Please expose TreeBuilder for ease of writing custom parsers

[garthk]

You don't expose the TreeBuilder to the supplied parser:

ElementTree.prototype.parse = function(source, parser) {
  if (!parser) {
    parser = get_parser(constants.DEFAULT_PARSER);
    parser = new parser.XMLParser(new TreeBuilder());
  }
  parser.feed(source);

Strikes me that makes it harder to get any work done.