I have thought to do such thing but I think there should be 2 different classes (`Delegating*` and `Delegating*Mixin`).
The goal of a `DelegatedList` is to hide the delegate and sometimes customize the behaviour. For this later case I don't think it's good to provide a way to get the `delegate`. For a `DelegatedListMixin` there's no over way than having an `abstract List<E> get delegate;`. I will make another PR with Mixins.