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chromium
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chromium/third_party/blink/web_tests/fast/events/resources/message-port-gc-closed-cloned-iframe.html 

<!DOCTYPE html>
<script>
window.onmessage = function(evt) {
    var port = evt.ports[0];
    port.start();
    port.postMessage("startedAndClosed");
    port.close();
}
</script>