String modeString()

Returns the mode value as a human-readable string, in the format "rwxrwxrwx", reflecting the user, group, and world permissions to read, write, and execute the file system object, with "-" replacing the letter for missing permissions. Extra permission bits may be represented by prepending "(suid)", "(guid)", and/or "(sticky)" to the mode string.

Source 

String modeString() {
  var permissions = mode & 0xFFF;
  var codes = const ['---', '--x', '-w-', '-wx', 'r--', 'r-x', 'rw-', 'rwx'];
  var result = [];
  if ((permissions & 0x800) != 0) result.add("(suid) ");
  if ((permissions & 0x400) != 0) result.add("(guid) ");
  if ((permissions & 0x200) != 0) result.add("(sticky) ");
  result
    ..add(codes[(permissions >> 6) & 0x7])
    ..add(codes[(permissions >> 3) & 0x7])
    ..add(codes[permissions & 0x7]);
  return result.join();
}