Public method for getting the resource's ID?

We have a few ways of getting the resource ID: `resource._getUid()` works, but the underscore makes it look like an internal method I shouldn't depend on. `resource.toJSON().id` also works, but it processes the entire data structure which I don't need. Finally, `resource.get('id')` doesn't work, which surprises me.

Is there a recommended and stable way of getting a resource object's ID?