anchor.diff:    }) {
       assert(selectedDate != null);
       assert(currentDate != null);
  +    assert(onChanged != null);
       assert(displayedMonth != null);
     }
   
  
positive.code:     double itemExtent
    }) : super(itemExtent: itemExtent) {
      assert(selectedDate != null);
      assert(lastDate.isAfter(firstDate));
    }
  
  
negative.code:   DatePickerMode _mode = DatePickerMode.day;
  
    void _handleModeChanged(DatePickerMode mode) {
      userFeedback.performHapticFeedback(HapticFeedbackType_VIRTUAL_KEY);
      setState(() {
        _mode = mode;
      });
    }
  
    void _handleYearChanged(DateTime dateTime) {
      userFeedback.performHapticFeedback(HapticFeedbackType_VIRTUAL_KEY);
      setState(() {
        _mode = DatePickerMode.day;
      });
      if (onChanged != null)
        onChanged(dateTime);
    }
  
    void _handleDayChanged(DateTime dateTime) {
      userFeedback.performHapticFeedback(HapticFeedbackType_VIRTUAL_KEY);
      if (onChanged != null)
        onChanged(dateTime);
    }
  