Talked to KK about this. He was not aware that changes were being made to the YUI code. 6 have been made in total since the update to v2.9 (not including this change). See JENKINS-23405.
Re minification... it's all obfuscated so changing by hand is a last resort imo. I tried an online YUI compressor. Output is formatted differently and the obfuscation is not exactly the same, but it seems to work (pushed to the same branch if you want to look). Might be dangerous though I think? Maybe we need to fork YUI2 as suggested by KK in JENKINS-23405.