CONSOLE WARNING: line 18: Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check https://xhr.spec.whatwg.org/.
Test for:

bug 40996: Progress event should not be fired during synchronous XMLHttpRequest;
bug 17502: Assertion failure when trying to restart a sync XMLHttpRequest as an async one from onreadystatechange.
 Step 1: Same origin request
readystatechange 4
load
loadend

Step 2: Cross origin request, disallowed
NetworkError: Failed to execute 'send' on 'XMLHttpRequest': Failed to load 'http://localhost:8000/xmlhttprequest/xmlhttprequest-sync-vs-async-assertion-failure.html'.

Step 3: Cross origin request, allowed
readystatechange 4
load
loadend

Step 4: Cross origin request, check that preflight isn't attempted
readystatechange 4
load
loadend

