JJ KasperandGitHub 9b09b92a14 Ensure adding _app/_document HMRs correctly (#28279) ...

This is a follow-up to https://github.com/vercel/next.js/pull/28227 to ensure `_app` and `_document` HMR correctly when you start the dev server and then add `_app` and `_document`. 

## Bug

- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [x] Errors have helpful link attached, see `contributing.md`

x-ref: https://github.com/vercel/next.js/issues/27888

2021-08-19 08:12:12 +00:00

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index.js

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