We'll do this everywhere eventually, but much easier to do it
incrementally:
- Use checker framework to default all field/methods/parameters to
@Nonnull.
- Start using ErrorProne[1] and NullAway[2] to check for possible null
pointer issues. I did look into using CheckerFramework, but it's much
stricter (i.e. it's actually Correct). This is technically good, but
is a much steeper migration path, which I'm not sure we're prepared
for yet!
[1]: https://github.com/google/error-prone
[2]: https://github.com/uber/NullAway
