
// be examined (most notably for which global proxy it was targeted at).
async function nextErrorEvent(windows) {
  let listener;
  let p = new Promise((resolve, reject) => {
    listener = (event) => { resolve(event); event.preventDefault(); };
  });
  for (let w of windows) {
    w.addEventListener('error', listener);
  }
  try {
    return await p;
  } finally {
    for (let w of windows) {
      w.removeEventListener('error', listener);
    }
  }
}
const supportsCustomizedBuiltinElements = (function() 