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Test Info:

<!DOCTYPE html>
<html>
<head>
<script>
class CustomElement extends HTMLElement {
constructor () {
super();
}
}
let element = document.createElement("custom-element");
customElements.define("custom-element", CustomElement);
customElements.upgrade(element);
const internals = element.attachInternals();
element = undefined;
SpecialPowers.forceGC();
SpecialPowers.forceCC();
internals.shadowRoot;
</script>
</head>
</html>