Can you elaborate this a bit?

How can I elaborate?

Run node build.js --copy-assets and look at the directory I mentioned. I shouldn't be generated

Not sure how else to explain it

These come from #26300
I guess they are supposed to replace most of core.es6.js which is still present in build.
I indeed wonder also why we have both in media/ as core.es6.js is recomposed there with their content.

I believe the aim was to conditionally load the es6 files on machines that support ES6 (basically everything we run core on at the moment out of the box). Can you explain why you think these files are invalid. I'm often using them to debug things (as it's the equivalent of the raw files used before babel transpiling)

The files are invalid because they are compiled to core.es6.js, that are only the plain source files for copyright reasons.

@wilsonge last time I checked, there was no such thing as a .es6 file extension.

Note .es6 vs .es6.js

Ahh sorry I understand now. Yup that's all invalid and rubbish indeed

Please test PR #31201.