spec/fixtures/api/close.html (10 lines of code) (raw):
<html>
<body>
<script type="text/javascript" charset="utf-8">
window.addEventListener('unload', function (e) {
require('fs').writeFileSync(__dirname + '/close', 'close');
}, false);
window.onload = () => window.close();
</script>
</body>
</html>