Are you using Flash or Flash Builder? Have you gone through the flex-config.xml file in your SDK/frameworks directory, one item in there seems like it might be what you're looking for. Try looking for the following
<!-- Use the ECMAScript edition 3 prototype based object model to allow dynamic overriding of prototype -->
<!-- properties. In the prototype based object model built-in functions are implemented as dynamic -->
<!-- properties of prototype objects (-strict is allowed, but may result in compiler errors for -->
<!-- references to dynamic properties) -->
<es>false</es>
and changing it to true. You can also disable just about every type of warning in there as well as turn on some that normally aren't enabled but sometimes should be.
It should be noted however that none of these "settings" actually work or do anything, and neither does unchecking "Enable Warnings" in the Flash Builder/Flex Compiler settings, or changing your mm.cfg, or even holding an "Advanced Flex Builder Topics" book against the wall with your butt while you punch your nuts into the sheetrock and pray for inspiration.
I just thought I'd point out their existence in case some day Adobe decides to tighten things up a bit and actually implement some of the stuff that they have a zillion settings all over the place for.