asDigit method
Turn char into a number representing a digit, or null if it doesn't
represent a digit in this locale.
Implementation
int? asDigit(String char) {
  var charCode = char.codeUnitAt(0);
  var digitValue = charCode - _localeZero;
  if (digitValue >= 0 && digitValue < 10) {
    return digitValue;
  } else {
    return null;
  }
}