anchor.diff:      double itemExtent
     }) : super(itemExtent: itemExtent) {
       assert(selectedDate != null);
  +    assert(onChanged != null);
       assert(lastDate.isAfter(firstDate));
     }
   
  
positive.code:         item = new Listener(
            onGestureTap: (_) {
              DateTime result = new DateTime(year, month, day);
              if (onChanged != null)
                onChanged(result);
            },
            child: new Container(
              height: 30.0,
  
negative.code: 
  import 'dart:async';
  
  import 'package:sky/mojo/activity.dart';
  import 'package:sky/theme/colors.dart' as colors;
  import 'package:sky/theme/typography.dart' as typography;
  import 'package:sky/widgets.dart';
  