chromium/src/third_party/blink/web_tests/http/tests/security/MessagePort/resources/attacker-frame.html

<script>

window.createClosure = function() {
    var MessageChannelConstructor = window.MessageChannel;
    var channel;
    return function() {
        return new MessageChannelConstructor;
    }
}
</script>
<p>Attacker iframe.</p>