anchor.diff:      this.lastDate
     }) : super(itemExtent: 50.0) {
       assert(selectedDate != null);
  +    assert(onChanged != null);
       assert(lastDate.isAfter(firstDate));
     }
     DateTime selectedDate;
  
positive.code:     double itemExtent
    }) : super(itemExtent: itemExtent) {
      assert(selectedDate != null);
      assert(lastDate.isAfter(firstDate));
    }
  
  
negative.code:   }) {
      assert(selectedDate != null);
      assert(currentDate != null);
      assert(onChanged != null);
      assert(displayedMonth != null);
    }
  
  