anchor.diff:      double itemExtent
     }) : super(itemExtent: itemExtent) {
       assert(selectedDate != null);
  +    assert(onChanged != null);
       assert(lastDate.isAfter(firstDate));
     }
   
  
positive.code:         key: new Key(label),
          onGestureTap: (_) {
            DateTime result = new DateTime(year, selectedDate.month, selectedDate.day);
            if (onChanged != null)
              onChanged(result);
          },
          child: new InkWell(
            child: new Container(
  
negative.code:   }) {
      assert(selectedDate != null);
      assert(currentDate != null);
      assert(onChanged != null);
      assert(displayedMonth != null);
    }
  
  