Fix IE8/IE9 bug that may hang cross-domain requests

When loading JS modules with StealJS from another domain, IE9 would not finish loading until I applied this patch.

This patch may have to go further upstream as I think it's in either SystemJS or es6-module-loader.

See following descriptions of bug and workaround:

http://stackoverflow.com/questions/7037627/long-poll-and-ies-xdomainrequest-object/9928073#9928073

http://social.msdn.microsoft.com/Forums/ie/en-US/30ef3add-767c-4436-b8a9-f1ca19b4812e/ie9-rtm-xdomainrequest-issued-requests-may-abort-if-all-event-handlers-not-specified?forum=iewebdevelopment