isFirstDayOfWeek method
Checks if day is in the first day of a week.
You can define first weekday (Monday, Sunday or Saturday) with
parameter firstWeekday. It should be one of the constant values
DateTime.monday, ..., DateTime.sunday.
By default it's DateTime.monday.
Implementation
bool isFirstDayOfWeek(DateTime day, {int firstWeekday = DateTime.monday}) {
assert(firstWeekday > 0 && firstWeekday < 8);
return isSameDay(firstDayOfWeek(day, firstWeekday: firstWeekday), day);
}