decode method
override
    Decodes the ASCII bytes (a list of unsigned 7-bit integers) to the
corresponding string.
If bytes contains values that are not in the range 0 .. 127, the decoder
will eventually throw a FormatException.
If allowInvalid is not provided, it defaults to the value used to create
this AsciiCodec.
Implementation
String decode(List<int> bytes, {bool allowInvalid}) {
  if (allowInvalid == null) allowInvalid = _allowInvalid;
  if (allowInvalid) {
    return const AsciiDecoder(allowInvalid: true).convert(bytes);
  } else {
    return const AsciiDecoder(allowInvalid: false).convert(bytes);
  }
}