The optional 2nd and 3rd arguments are not yet implemented.
This assumes that `this` is the Array constructor and doesn't yet
implement the more general behavior in the ES6 spec that allows
transferring this method to other constructors.
With the addition of symbol keys, work can now be done on starting to
implement the well-known symbol functionality. The most important of
these well-known symbols is by far Symbol.iterator.
This patch adds IteratorPrototype, as well as ArrayIterator and
ArrayIteratorPrototype. In the future, sometime after StringIterator has
also been added, this will allow us to use Symbol.iterator directly in
for..of loops, enabling the use of custom iterator objects. Also makes
adding iterator support to native objects much easier (as will have to
be done for Map and Set, when they get added).
