The compiler error/warning sounds very similar to my http://bugs.adobe.com/jira/browse/SDK-25720 bug, "Compiler warning when using an mx:List in Flex 4 app w/ Halo theme".
Please vote/comment in the JIRA bug if it is important to you.
Yes, very irritating. "Only a warning" isn't the whole story, it's more than just an annoyance, it can potentially make all warnings useless.
Imagine cases where you have multiple warnings (often FB warnings turn up for EACH component resulting in many repeats, design mode is particularly bad with this). Having a ton of warnings that "aren't really warnings" obscures REAL warnings that we'd want to do something about (and makes errors harder to notice at first--you're so used to always seeing lines under the problems tab that you don't always notice a new error that just popped up). In the worst case scenario a valid warning that would have prevented a bug is missed and something goes to QA or production that should have been caught at compile time...
This wouldn't be nearly as annoying if we had some decent way of filtering these warnings from the problems tab. The current filter isn't good enough and only seems to allow you to filter out one message due to the way it works, but sometimes we see more than one type of "bogus" warning.
Voted and added to comments. I wonder if there's a feature request out there to improve warning filter capabilities? I also wonder why nobody complained about this before, it seems pretty easily visible. Is it the norm for people to just always ignore warnings? This isn't a good practice, warnings tend to exist for a reason.
(Btw, this is part of my reasoning for the claim that Halo seems to have become a "second class citizen" component set - hopefully Adobe can do something about this.)