Fix open source yarn start
Fix open source yarn start

Summary: The fb-stubs transform is getting applied when it shouldn't. Fix it by only applying it if src/fb exists.

Reviewed By: danielbuechele

Differential Revision: D13236217

fbshipit-source-id: 940def377839d16f34c49504948cb062916a3d6a

facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper