Operator new doesn't set is-value for custom elements #35929
Description
let klass = eval(`(class extends HTMLAnchorElement {})`);
customElements.define("my-a", klass, { extends: "a" });
let foo = new klass();
let bar = document.createElement("a", {is: "my-a"});foo's is value is None, bar's is value is Some(Atom('my-a' type=inline)).
This is causing around 100 tests in custom-elements/builtin-coverage.html to fail.