Embroider + FastBoot file override compatibility issue

[Windvis]

I while ago I ran into an issue when enabling Embroider in a FastBooted app because some dependencies weren't node compatible. It was then suggested to use FastBoot's file overwrite feature to create a noop modifier to bypass the problem. This seemed to work at first, but we ran into issues when deploying this to production and reverted back to the classic build system.

I just revisited the issue again and found the conditions that are needed to reproduce it:

• The app must be using Embroider
• The app must be built for production
• You must be overwriting a file with a fastboot specific version
• The file that you are overwriting must be importing .css
• No other files must be importing css (including dependencies)

Once all conditions are met, FastBoot will throw the following error when accessing the app:

Error: Could not find module `fastboot-embroider-repro/app` imported from `~fastboot/app-factory`

The problem is resolved by also importing the css in the overwritten file.

So it seems something goes wrong when bundling the imported css for production if the import happens in a file that is overwritten for FastBoot.

Reproduction: https://github.com/Windvis/fastboot-file-override-embroider-reproduction