LayoutTests/http/tests/contentextensions/script-onerror.html

<script>
    if (window.testRunner) {
        testRunner.dumpAsText();
        testRunner.waitUntilDone();
    }

    var s = document.createElement('script');
    s.src = "should-be-blocked.js";
    s.onerror = function () {
        if (window.testRunner) {
            console.log("onerror was correctly called on the next runloop cycle.");
            testRunner.notifyDone();
        }
    };

    document.getElementsByTagName('head')[0].appendChild(s);
    console.log("Done executing script.  Waiting for onerror callback.");
</script>