startOfWeek method
Start of week. firstWeekday 1 = Monday, 7 = Sunday (default Monday).
Returns the DateTime at 00:00:00 on the first day of the week.
Implementation
@useResult
DateTime startOfWeek({int firstWeekday = DateTime.monday}) {
final int delta =
(weekday - firstWeekday + DateConstants.daysPerWeek) % DateConstants.daysPerWeek;
return DateTime(year, month, day - delta);
}