Making that simple is not necessarily limited to prototype-based languages. Another (as good as) dead language of Apple's design, Dylan, is class-based, but has a 'each-subclass' modifier that makes a slot magically appear new in each subclass. You would have to design your polygon class for that, though; it is not something that triangle could magically add.
It is too bad someone hasn't gone back a released a retooled, s-expr version of Dylan. It was an interesting language. People should read the manual (infix) just to see how they did macros.
