anchor.diff:      this.lastDate
     }) : super(itemExtent: 50.0) {
       assert(selectedDate != null);
  +    assert(onChanged != null);
       assert(lastDate.isAfter(firstDate));
     }
     DateTime selectedDate;
  
positive.code:       case DatePickerMode.day:
          picker = new MonthPicker(
            selectedDate: selectedDate,
            onChanged: onChanged,
            firstDate: firstDate,
            lastDate: lastDate,
            itemExtent: _calendarHeight
  
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';
  