Standard builtins' prototypes and toString

[C. Scott Ananian]

It would be slightly more "JavaScripty" to have
Date.prototype.[[DateValue]] exist, and be set to the epoch or some
such.

This problem actually seems to be an artifact of the way that
[[Construct]] works in ES6 -- but the takeaway is that prototypes of a
class are not themselves instances of the class.  It's not surprising
that methods of the class thus don't work on the prototype.  I'd vote
WONTFIX.
  --scott