[v4] Fast Refresh should not be enabled for old ReactDOM versions

[gaearon]

If Fast Refresh is unavailable then we should fall back to full reloads. Currently, if you use CRA 4 with an old version of React (e.g. 15) it just doesn't do anything on edit.

[n3tr]

I'm happy to take a look at this if no one else is currently looking.

[stale]

This issue has been automatically marked as stale because it has not had any recent activity. It will be closed in 5 days if no further activity occurs.