toISOTime method
Implementation
String toISOTime() {
final hh = _pad(hour, 2);
final mm = _pad(minute, 2);
final ss = _pad(second, 2);
final ms = _pad(millisecond, 3);
final off = isUtc ? 'Z' : _tzOffsetWithColon(timeZoneOffset);
return '$hh:$mm:$ss.$ms$off';
}