Conforming to M3

I depend on dart-xml on one of my projects, and unfortunately, the latest breaking changes in Collection had a big impact on XmlCollection.

I am very sure I could do this much more elegantly than I did in the end. Maybe you will want to ignore this pull request and update the library in a more meaningful way.

EDIT: I also found my handle in the AUTHORS file. Thanks! I went ahead and substituted the handle with my name.