# File lib/rubygems/package.rb, line 264
    def mkdir(name, mode)
      raise ClosedIO if @closed
      name, prefix = split_name(name)
      header = TarHeader.new(:name => name, :mode => mode, :typeflag => "5",
                             :size => 0, :prefix => prefix).to_s
      @io.write header
      nil
    end