Ability to influence XMLHttpRequest object creation

[t3hmrman]

While using fetch to write a FirefoxOS app, I've realized that I need to be able to pass in creation params to the underlying XMLHttpRequest.

I apologize if there is already a way to do this that I missed, but in my case I need the XMLHttpRequest creation (on line 245) to actually look like this:

var xhr = new XMLHttpRequest({mozSystem: true});

https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest
(See the CORS section)

[dgraham]

The XMLHttpRequest instance is an implementation detail that is not included as part of the public Fetch API.