Ok, so this can be reproduced with `mvn clean verify -Pbuild-individual-bundles,api-check` from eclipse.platform.text/org.eclipse.ui.workbench.texteditor (using your commit).
I see multiple possible causes, all of them being actual bugs in a place or another:
1. The error seems simply wrong as you mention: with is a known element and there is no reason to report it as unknown, so it seems like under some circumstances, the PDE Builder for plugin.xml can return erroneous contents; or that the way it's configured for API Analysis is faulty
2. in the org.eclipse.pde.prefs for that bundle, the "compilers.p.unknown-element" error type is configured to 0 (which, unless I'm mistaken means ignore), however the error is reported as an error by the builder during API Analysis. Here again, eiher the issue is in the builder itself, or in API Analysis messing up the configuration.
Anyway, the symptom and the 2 possible causes need deeper investigation. Can you please report a bug about it as we have the luxury of having a reproducer ;)
Cheers,
