Checks whether flag_string contain a --flagfile=<foo> directive.

def _is_flag_file_directive(self, flag_string):
    """Checks whether flag_string contain a --flagfile=<foo> directive."""
    if isinstance(flag_string, type('')):
      if flag_string.startswith('--flagfile='):
        return 1
      elif flag_string == '--flagfile':
        return 1
      elif flag_string.startswith('-flagfile='):
        return 1
      elif flag_string == '-flagfile':
        return 1
      else:
        return 0
    return 0