ment = document.querySelector('custom-element');
  const calls = [];
  let flushA = undefined;
  let flushB = undefined;

  customElements.polyfillWrapFlushCallback(function callbackA(flush){
    calls.push('callbackA');
    flushA = flush;
  });
  customElements.polyfillWrapFlushCallback(function callbackB(flush){
    calls.push('callbackB');
    flushB = flush;
  });

  assert(!element.upgraded);
  assert.deepEqual(calls, []);

  customElements.define('custom-element', class extends HTMLElement {
    cons