Does anyone know of a simple, standalone cross-domain XHR interface that allows for the ability to swap between a real XHR implementation and a cross-domain, iframes-based implementation?

I know Dojo and DWR, among others, have them, but they are all part of a larger framework. Because this is going to be used with Selenium, which needs to avoid polluting common popular JS namespaces, I'd prefer to find something that is standalone.

I'm very surprised one doesn't pop up when I search google.