This is needed for flat builds. It also lets us get rid of a bunch
of special cases in the build scripts.
It also allow us to just copy the source files into React Native
instead of having to build first to resolve the special cases.
This just configures flow to be checked and fixes our existing
Flow typed files.
Possible enhancements:
Export .js.flow declarations from the build. Unclear whether this
will be a supported workflow in the future or not, so let's wait
on that.
We should fail builds and CI on Flow errors.
Ideally we should lint for Flow style guides (like no space before
colon).
