add test for update selection by modifying bound field

I tried to bind the `selection` attribute to a field in my model 
```html
<core-list-dart id="menu"
                      data="{{model.filtered}}"
                      selection="{{model.activeItem}}"
                      class="children" flex height="16">
```
and expected when I assign an item to `model.activeItem` that `core-list-dart` shows this item as selected but it doesn't.
The PR contains a test which verifies this behavior
as for the cast - I'm OK either way - The check is done only in checked mode, so using 'as' will make sure we do the check even when running the test in unchecked mode.