It's possible to hide that as well, funnily enough, by also overwriting Object.getOwnPropertyDescriptor (and similar tricks). As far as I know, it's theoretically possible to use this trick to completely 'sandbox' some code so that there's no way it can detect certain functions being overwritten (by overwriting all functions such as Object.getOwnPropertyDescriptor, Function.toString, etc. and making them hide the overwritten functions, including themselves).

For some more information: http://randomwalker.info/publications/ad-blocking-framework-...