anchor.diff:  
     @Observe('items.*')
     void itemsChanged() {
  -    if (storageId != null) {
  +    if (storageId != null && items != null) {
         storage[storageId] = JSON.encode(items);
       }
     }
  
positive.code:   TodoInput get _newTodo => $['new-todo'];
  
    @eventHandler
    bool itemsIsNotEmpty() => items.isNotEmpty;
  
    @eventHandler
    bool isZero(int value) => value == 0;
  
negative.code:   bool anyItems;
  
    @Property(observer: 'itemsChanged')
    List<Todo> items;
  
    @Property(observer: 'storageIdChanged')
    String storageId = 'storage';
  