Cannot subclass builtin types

[lance]

Array has already been addressed, but other builtin types such as Date, String, Boolean and Number cannot be subclassed. This is a low priority, because it's not really clear what you'd do with a subclass of Number or Boolean, because successfully subclassing them in V8 you end up with a semi-useless object. E.g. you can't subclass Number and then add an instance of that subclass to a real number.