How to use the lit-element.LitElement.prototype function in lit-element

import "../../resources/ha-style";
import registerServiceWorker from "../../util/register-service-worker";
import { DEFAULT_PANEL } from "../../common/const";

import HassBaseMixin from "./hass-base-mixin";
import AuthMixin from "./auth-mixin";
import TranslationsMixin from "./translations-mixin";
import ThemesMixin from "./themes-mixin";
import MoreInfoMixin from "./more-info-mixin";
import SidebarMixin from "./sidebar-mixin";
import { dialogManagerMixin } from "./dialog-manager-mixin";
import ConnectionMixin from "./connection-mixin";
import NotificationMixin from "./notification-mixin";
import DisconnectToastMixin from "./disconnect-toast-mixin";

LitElement.prototype.html = litHtml;

const ext = (baseClass, mixins) =>
  mixins.reduceRight((base, mixin) => mixin(base), baseClass);

export class HomeAssistant extends ext(PolymerElement, [
  AuthMixin,
  ThemesMixin,
  TranslationsMixin,
  MoreInfoMixin,
  SidebarMixin,
  DisconnectToastMixin,
  ConnectionMixin,
  NotificationMixin,
  dialogManagerMixin,
  HassBaseMixin,
]) {