[fbjs] Restore flow/lib/dev.js (#186)
[fbjs] Restore flow/lib/dev.js (#186)

This is primarily for direct consumers who want to use `__DEV__` in
their code and run flow on that code before the `__DEV__` transform is
run.
chicoxyzzy/fbjs,cpojer/fbjs,yungsters/fbjs,facebook/fbjs