[V4] Flash of unstyled content during Fast Refresh

Created by: gaearon

This doesn't repro every single time, but I occasionally see a flash of unstyled content after introducing and fixing a build error.

Here is a slowed down gif showing it:

Individual unstyled frames (this is in the default CRA template):

So it looks like we see unstyled app first (CSS getting applied too late?) and then we see a bizarre error overlay with terminal codes that then instantly disappears.