Re: "var" declarations shadowing properties from Window.prototype

One incompatibility in current implementations is that
Object.getPrototypeOf(window) in Opera is Object.prototype whereas the rest
have something like Window.prototype (Window doesn't exist in Opera).

An interesting consequence of mutable __proto__ and having the global
variable record be a "regular" object is that mutating its __proto__ has
the same net effect as using `with`.