Summary: Installed plugins loaded and make it impossible to debug plugin changes. We need to disable them by default for `yarn start`. To debug plugin installation and auto-update, need to use command `yarn start --plugin-auto-update` instead. Reviewed By: passy Differential Revision: D25639345 fbshipit-source-id: 99b59a90dc09ac33a3604487fe501774e5e6fc39