XMLHttpRequest isn't rooted while in flight

[Chris Toshok]

osx-test/hello-osx.js has this block:

            this.xmlhttp = new XMLHttpRequest();
            this.xmlhttp.onreadystatechange = () => {
        console.log ("readyState == " + this.xmlhttp.readyState);
        if (this.xmlhttp.readyState === 4) {
                    console.log ("woohoo!");
            console.log (this.xmlhttp.responseText);
        }
            };
        console.log (this.xmlhttp.onreadystatechange);
            this.xmlhttp.open('GET', 'http://www.google.com/', true);
            this.xmlhttp.send();

'xmlhttp' should be a local variable, but if you make it one we crash, since the .send() doesn't cause it to be rooted by the GC. The GC frees it and we crash when attempting to call the onreadystatechange callback.