anchor.diff:    }) {
       assert(selectedDate != null);
       assert(currentDate != null);
  +    assert(onChanged != null);
       assert(displayedMonth != null);
     }
   
  
positive.code: 
  import 'dart:async';
  
  import 'package:sky/theme/colors.dart' as colors;
  import 'package:sky/theme/typography.dart' as typography;
  import 'package:sky/widgets.dart';
  
negative.code:     double itemExtent
    }) : super(itemExtent: itemExtent) {
      assert(selectedDate != null);
      assert(onChanged != null);
      assert(lastDate.isAfter(firstDate));
    }
  
  