Function polyfill

src/polyfills/custom-elements.js:651–758

* Does the polyfilling. * @param {!Window} win

(win)

 * @param {!Window} win
 */
function polyfill(win) {
  const {Element, HTMLElement, document} = win;
  const {createElement} = document;

  const registry = new Registry(win);
  const customElements = new CustomElementRegistry(win, registry);

  // Expose the custom element registry.
  // Object.getOwnPropertyDescriptor(window, 'customElements')
  // {get: ƒ, set: undefined, enumerable: true, configurable: true}
  Object.defineProperty(win, 'customElements', {
    enumerable: true,
    configurable: true,
    // writable: false,
    value: customElements,
  });

  // Have to patch shadow methods now, since there's no way to find shadow trees
  // later.
  const elProto = Element.prototype;
  const {attachShadow, createShadowRoot} = elProto;
  if (attachShadow) {
    /**
     * @param {{mode: string}} unused
     * @return {!ShadowRoot}
     */
    elProto.attachShadow = function (unused) {
      const shadow = attachShadow.apply(this, arguments);
      registry.observe(shadow);
      return shadow;
    };
    // Necessary for Shadow AMP
    elProto.attachShadow.toString = function () {
      return attachShadow.toString();
    };
  }
  if (createShadowRoot) {
    /** @return {!ShadowRoot} */
    elProto.createShadowRoot = function () {
      const shadow = createShadowRoot.apply(this, arguments);
      registry.observe(shadow);
      return shadow;
    };
    // Necessary for Shadow AMP
    elProto.createShadowRoot.toString = function () {
      return createShadowRoot.toString();
    };
  }

  /**
   * You can't use the real HTMLElement constructor, because you can't subclass
   * it without using native classes. So, mock its approximation using
   * createElement.
   * @return {!ElementOrigDef}
   */
  function HTMLElementPolyfill() {
    const {constructor} = this;