Devtools: Fix build-for-devtools

[eps1lon]

Stack:

1. Devtools: Streamline getting extension from branch #28975
2. Devtools: Fix build-for-devtools #28976 <--- You're here
3. DevTools: Fix inspecting components with multiple reads of the same Context in React 17 #28974

Summary

Some devtools code imports from shared/ this could result in shared/FeatureFlags becoming part of the module graph as it did with #28813. Without this fix, we'd have a bare __EXPERIMENTAL__ identifier that would result in react_devtools_backend_compact.js:3724 Uncaught ReferenceError: __EXPERIMENTAL__ is not defined in the final build.

We already hardcoded __EXPERIMENTAL__: true in the frontend Webpack config so I'm doing the same for the backend.

How did you test this change?

• Use react-devtools-chrome-extension.zip from this branch and no longer observe ReferenceError when inspecting a React app

[react-sizebot]

Comparing: c44e9d1...0618945

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.66 kB	6.66 kB	+0.11%	1.82 kB	1.82 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	492.61 kB	492.61 kB	=	87.88 kB	87.88 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.67 kB	6.67 kB	+0.11%	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	498.86 kB	498.86 kB	=	88.92 kB	88.93 kB
facebook-www/ReactDOM-prod.classic.js	=	591.22 kB	591.22 kB	=	103.96 kB	103.96 kB
facebook-www/ReactDOM-prod.modern.js	=	567.44 kB	567.44 kB	=	100.36 kB	100.36 kB
test_utils/ReactAllWarnings.js	Deleted	64.26 kB	0.00 kB	Deleted	16.02 kB	0.00 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
test_utils/ReactAllWarnings.js	Deleted	64.26 kB	0.00 kB	Deleted	16.02 kB	0.00 kB

Generated by 🚫 dangerJS against 0618945