Standard builtins' prototypes and toString

[Till Schneidereit]

While working on changing Date.prototype to be a plain object in
SpiderMonkey, we realized that there's an issue: the way things are specced
now, `alert(Date.prototype)` will throw, because `Date.prototype.toString`
isn't generic. The same applies for all builtins with non-generic
`toString` prototype functions.

To resolve this, I propose changing these `toString` to first check if the
`this` value is %FooPrototype% (e.g., %DatePrototype% in the case at hand)
and return the result of the equivalent of calling the original
Object.prototype.toString.

I'm not sure if that is enough to cover subclasses of these builtins. Will
calling `toString` on the prototype of `class MyDate extends Date{}` still
throw? If so, that would at least not be a backwards-compatibility concern,
but it's probably also not desirable.


till
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