HtmlNet needs to support CORS in IE9: try XDomainRequest when XMLHttpRequest fails

[GoogleCodeExporter]

In order to issue cross origin XHRs in IE9, you must use XDomainRequest instead 
of XMLHttpRequest. All other browsers let you just XMLHttpRequest.

Currently, HtmlNet uses XMLHttpRequest exclusively, which means that cross 
origin requests will fail in IE.

We should either update HtmlNet to fall back to XDomainRequest, in the same way 
that HtmlAssetManager already does --or-- just fix 
http://code.google.com/p/google-web-toolkit/issues/detail?id=5715.

Original issue reported on code.google.com by fredsa@google.com on 8 Jul 2011 at 5:35