operator []= method
Implementation
void operator []=(PeriodUnits unit, int value) {
// if ((unit.value & PeriodUnits.allDateUnits.value) != 0)
// {
// Preconditions.checkArgumentRange('value', value, int.MinValue, int.MaxValue);
// }
if (_indexSetterFunctionMap.containsKey(unit)) return _indexSetterFunctionMap[unit]!(this, value);
throw ArgumentError('Indexer for PeriodBuilder only takes a single unit');
}