s/mini/template.html#L52
        if (window.CustomElements && window.CustomElements.upgradeSubtree) {
          window.CustomElements.upgradeSubtree(stamped);
        }
      }

      return stamped;
    }
  };

  Object.getOwnPropertyNames(TestFixtureExtension)
    .forEach(function (property) {
      Object.defineProperty(
        TestFixturePrototype,
        property,
        Object.getOwnPropertyDescriptor(TestFixtureExtension, property)
      );
    });

  try {
    if (window.customElements) {
      