Summary:
Following up on fb7fe2d4e8: when <Modal> is used in dev mode, it renders `<AppContainer>` to wrap the children so that the element inspector can show up correctly. In that scenario, we need pass the `rootTag` over the `<AppContainer>` so that the children can read the rootTag correctly. Otherwise, the children of <Modal> will see it as undefined.
With this, AppContainer can then declare `rootTag` as a required prop, as it should have been.
Note that this only affects DEV build because there's no AppContainer wrapping otherwise.
Reviewed By: jingc
Differential Revision: D4204011
fbshipit-source-id: 80edbc8d351d983786e6fc3c68dfa65a71b1ed3c
4530da87e2