Seems cleaner and more readable. (With apologies to Hixie.)
There are non-readonly unforgeable properties.
Well, depending on how exactly you want to define Location, I suppose.
But in general there is no reason to restrict unforgeable to readonly properties; it can make sense for operations and writable properties too, as the Location example shows.
Ah yeah. I forgot we put it on the interface object there. Mkay.