add method
Implementation
void add(DateTime date, T event) {
final eventsOfDate = events[date];
if (eventsOfDate == null)
events[date] = [event];
else
eventsOfDate.add(event);
}
void add(DateTime date, T event) {
final eventsOfDate = events[date];
if (eventsOfDate == null)
events[date] = [event];
else
eventsOfDate.add(event);
}