third_party/WebKit/LayoutTests/custom-elements/spec/resources/custom-elements-helpers.js

function create_window_in_test(t) {
  let p = new Promise((resolve) => {
    let f = document.createElement('iframe');
    f.srcdoc = '';
    f.onload = (event) => {
      let w = f.contentWindow;
      t.add_cleanup(() => f.remove());
      resolve(w);
    };
    document.body.appendChild(f);
  });
  return p;
}

function test_with_window(f, name) {
  promise_test((t) => {
    return create_window_in_test(t)
    .then((w) => {
      f(w);
    });
  }, name);
}