[michaelmartin]

Hadn't seen the union approach in #2 before, I like that a lot. The 2 named properties are intuitive to other developers reading it, but now we also get compile-time safety that the combinations they use are valid. Thanks!

For anyone else that this was new to, Flow has documented these as "disjoint unions": https://flow.org/en/docs/types/unions/#toc-disjoint-unions