Summary:
When running `yarn` it compiles all the packages files into `build` folders. We should probably ignore this to avoid them ending up checked-in on the repo.
Closes https://github.com/facebook/metro/pull/149
Reviewed By: rafeca
Differential Revision: D7229013
Pulled By: jeanlauliac
fbshipit-source-id: fbbc478fa1cda78231620e2ff0fa8e02539f30ed