As stated on the Umbrello download site http://uml.sourceforge.net/download.php, recent versions no longer depend on flex. I assume that the dependency can be removed from the ebuild, unless it serves some other purpose. Reproducible: Always Steps to Reproduce: 1. 2. 3.
Removed from umbrello 3.4.0_beta2. It will stay in kdesdk since it's a dep of kbabel.