For now, unit tests are attempting to reuse an existing DisplayListBuilder after they Build a DisplayList from it as they were written that way using MockCanvas. This is a really muddy way to perform the tests and doesn't match the way that the builders are used in real-world code. Additionally, there is a bunch of code in Build() right now to make that possible that we really shouldn't need.

DisplayListBuilder should be re-spec'd to not allow use after Build and we should adjust the tests accordingly.