isHex function
Implementation
bool isHex(char) {
final charCode = char.codeUnitAt(0);
return (isDigit(char) ||
(charCode >= 'a'.codeUnitAt(0) && charCode <= 'f'.codeUnitAt(0)) ||
(charCode >= 'A'.codeUnitAt(0) && charCode <= 'F'.codeUnitAt(0)));
}