Number.floor appears to be a valid function but is not

We recently had a problem where when compiling using FB and debugging on devices works fine. But when we built the app using Ant, it was crashing on Android devices. After debugging that, we found that Number.floor was not a function.

I cannot find any documentaiton that says Number.floor was ever a function in ActionScript/Flex/AIR.

So why does Flash Builder seem to think it's valid, let us compile it, and even run it on the device?! Is there some 'even more strict' compiler option I need to check?