code: public static String asPath(final String directory, final String filename) {
          return directory.concat(separator).concat(filename);
      }
docs: Inserts the platform-specific filesystem path separator between
  {@code directory} and {@code filename} and returns the resulting string.
  
  @param directory Non-null string
  @param filename Non-null string
  @return String following the format
  {@code directory<path_separator>filename}