An earlier version of Tree exposed attributes for `rootNode` and `children` that simply acted as getters for the properties for the sake of people who enjoy using abstractions for things that don't need abstractions.

But then I thought that maybe those people should learn not to use unnecessary abstractions, and removed them.

There's plenty of prior art in the library for Base-based constructors accepting config properties that don't become attributes. I think we need to stop defaulting to everything being an attribute for no reason other than that it's the YUI way.