Summary:
Having previously run `yarn build` can otherwise
cause your devserver to stop working.
Caveat: If you have a Flipper checkout and it's called neither `sonar` nor `flipper`, you're still screwed.
Reviewed By: danielbuechele
Differential Revision: D14364984
fbshipit-source-id: 1652c94a95b1dcb6d34bb9040a9cd4cd70c6df55