How to do it...

Open up stopwatch.dart to get started:

1. The first thing you are going to do is make the timer a bit more precise. Seconds are not a very interesting value to use for stopwatches. 
2. Use the refactoring tools to rename the seconds property to milliseconds. We also need to update the onTick, _startTimer, and _secondsText methods:

int milliseconds = 0;

void _onTick(Timer time) {
  setState(() {
    milliseconds += 100;
  });
}
void _startTimer() {
  timer = Timer.periodic(Duration(milliseconds: 100), _onTick);
  ...
}
String _secondsText(int milliseconds) {
  final seconds = milliseconds / 1000;
  return '$seconds seconds';
}

3. Now, let's add a laps list so that we can keep track of the values for each lap. We're going to add to this list every time the user taps a lap button. 
4. Add this property to the top of the StopWatchState class, just under the declaration of the timer:

final laps = <int>[];

5. Create a...