React version: 19.2.0

Steps To Reproduce

1. Clone the latest project from React Github.
2. Run yarn to install dependencies.
3. Run yarn build react/index,react-dom/index --type=UMD.

The current behavior

I did not get the expected build output. According to the documentation, the build folder should contain UMD files that are referenced in fixtures/packaging/babel-standalone/dev.html, but after inspecting that file, I noticed that the paths and files mentioned there were not generated during the build.

The expected behavior

The build folder should contain a UMD folder, where the UMD development files should reside.