When using ResourceLoader's debug mode, .js files are fetched directly from their home location in the resources directory.
On Wikimedia deployments this would be in /w/resources; like the old extensions dir this'll need to be either symlinked in so the files can be exposed, or else overridden with a reference to bits.wikimedia.org.
[Note that at the moment debug mode is additionally broken on test2.wikipedia.org due to bug 27321 / bug 27322. With the fix, it'll try actually loading the /w/resources files and will then fail due to them being missing.]
Version: unspecified
Severity: enhancement