decode method
override
    Decodes the UTF-8 codeUnits (a list of unsigned 8-bit integers) to the
corresponding string.
If the codeUnits start with the encoding of a
unicodeBomCharacterRune, that character is discarded.
If allowMalformed is true, the decoder replaces invalid (or
unterminated) character sequences with the Unicode Replacement character
U+FFFD (�). Otherwise it throws a FormatException.
If allowMalformed is not given, it defaults to the allowMalformed that
was used to instantiate this.
Implementation
String decode(List<int> codeUnits, {bool? allowMalformed}) {
  // Switch between const objects to avoid allocation.
  Utf8Decoder decoder = allowMalformed ?? _allowMalformed
      ? const Utf8Decoder(allowMalformed: true)
      : const Utf8Decoder(allowMalformed: false);
  return decoder.convert(codeUnits);
}