What should Map iteration do?

[Thaddee Tyl]

Related:

I cannot find the reason why sets and maps were harmoni'ed with
classes[1]. They do support `mySet["foo"]` for `mySet.has("foo")` and
`for (something of mySet)` for iteration, do they not? Classes don't
have operator overloading, so using classes makes it harder to see the
usual syntax form.

Shouldn't we use proxies instead?

  [1] http://wiki.ecmascript.org/doku.php?id=harmony:simple_maps_and_sets