So I spent about 30 minutes fighting this before I posted, but figured it out right afterwards. my "use namespace mx_internal"; was inside my class declaration, apparently it needs to be outside of it.
This sounds like a compiler bug. It should have been OK to put the 'use namespace mx_internal' directive inside the class. If you could reduce this problem to a simple case and file a bug, that would be very helpful.
Adobe Flex SDK Team
Thank you for posting your solution. I wish more people would do the same! Good job
I found a bug regarding this posted in bugs.adobe.com, but for the life of me I can't find it again.
I'm happy to post any answers I find to my own problems. It's not uncommon for me to forget a workaround and have to go back to my own post to find the solution.