update method
void
update()
Will be called by the internal timer, or may be called at any time.
Implementation
void update() {
if (startTime == null) throw Exception('Must call `start()` before calling this.');
var now = DateTime.now();
var diff = now.difference(startTime!).inMilliseconds;
if (diff < 0) throw Exception('Back in Time: Current time is before the start time!');
frameCount = framerate.realSecondsToFrames(diff / 1000);
}