I have a concern with these, which isn't 100% related to this thread, but
is on the topic of iterators.

So, in Dart, we have this nice option of basically saying `if (OBJ is
Iterable)`, and we'll be able to know easily if an object conforms to that
particular interface.

Admittedly, this construct makes a lot more sense in Dart than it does in
Javascript, but it would be nice to have some kind of helper which will
determine if
an object adheres to the Iterable interface. For instance,
`Object.isIterable() => true` if the interface is present in the object's
prototype.

It would be nice to have a convenience function for this largely because it
feels like the VM should have an easier time knowing if an object is
iterable or not, and may be able to cache its knowledge of the Iterable
status of an object, for performance reasons.

Maybe it would be good if a helper function like this would also determine
that the return value of `@@iterator()` also conforms to the Iterator
interface, or maybe that's not super important. But in any case, I could
see utilities like this being very helpful.

Maybe that ship has sailed, but if something like that would hurt the
harmony status of iterators, I would be interested in hearing the specific
reasons against it.
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I have a concern with these, which isn't 100% related to this thread, but
is on the topic of iterators.

So, in Dart, we have this nice option of basically saying `if (OBJ is Iterable)`, and we'll be able to know easily if an object conforms to that
particular interface.

Admittedly, this construct makes a lot more sense in Dart than it does in
Javascript, but it would be nice to have some kind of helper which will
determine if an object adheres to the Iterable interface. For instance,
`Object.isIterable() => true` if the interface is present in the object's prototype.

It would be nice to have a convenience function for this largely because it
feels like the VM should have an easier time knowing if an object is
iterable or not, and may be able to cache its knowledge of the Iterable
status of an object, for performance reasons.

Maybe it would be good if a helper function like this would also determine
that the return value of `@@iterator()` also conforms to the Iterator
interface, or maybe that's not super important. But in any case, I could
see utilities like this being very helpful.

Maybe that ship has sailed, but if something like that would hurt the
harmony status of iterators, I would be interested in hearing the specific
reasons against it.