Add flags and dead code elimination

Created by: gaearon

Hacks like https://github.com/facebookincubator/create-react-app/pull/52 are frustrating. We need a system that lets us write stuff like

if (__REPO__) {
  var path = ...
} else if (__DEPENDENCY__) {
  var path = ...
} else if (__EJECTED__) {
  var path = ...
}

and eliminate all other branches on eject.