That is actually a flash player thing, not a flex unit thing, just FYI.
Ah, OK... So, you are saying that all stacktraces produced in the Flash player include these paths, not just stacktraces from FlexUnit? I guess Adobe figures most users will be using release players and will never see them.
Thanks for clueing me in.
Sure, so, on the technical details side what is happening is that when actionscript code is built using the flex compiler, you can choose to include debug data or not. If you don't compile with debug you don't get cod hinting for the libs, etc. so most people compile as debug throughout development. The consequence of the debug compile though is that it includes all of this extra info, which flash player then spits out in the stack trace.
Here is a bug regarding these issues:
For this release we chose to provide the raw stack trace. In future version we may provide an optional filter which will reduce some of the noise, these paths potentially included.