Fix imported CSS with FastBoot

[simonihmig]

When importing CSS from a JS module in a FastBoot enabled app, Embroider would emit something like this into index.html:

<fastboot-script src="/assets/chunk.957ca35b25015b34de4d.js"></fastboot-script>
<fastboot-script src="/assets/chunk.96eb691090beaf949370.css"></fastboot-script>

This makes FastBoot try to parse and evaluate CSS as JavaScript, which obviously fails.

I tried to add a test case to fastboot-app, but this fails due to even more FastBoot/CSS issues (#1049)...

[ef4]

I guess I can see how it's possible to introduce "fastboot-only CSS", since you can import css files into JS modules that are Fastboot-only.

And I guess this PR is implying that that CSS will never do anything, and it will be dropped?

[simonihmig]

I guess I can see how it's possible to introduce "fastboot-only CSS", since you can import css files into JS modules that are Fastboot-only.

Actually I am not importing it from a FastBoot-only module. It is a regular module (so browser + FastBoot), but one that is lazily loaded (from a splitted route). When rendering that route in FastBoot, those lazy chunks are added with <fastboot-script> for FastBoot to process them eagerly AFAIU, right? And that breaks for CSS.

You can see this in the reproduction I used for #1049, the import-app-fastboot branch. I have the changes of this PR applied as a patch, but without it you should see the same error there. It is this lazy route that imports CSS and makes FastBoot fail.