Combining lit-html with Polymer3

lit-mixin.js

import { render as litRender } from 'lit-html/lit-html.js'

export const LitMixin = (superClass) => class extends superClass{
  static get properties() { return {} }

  static get observers() {
    const props = Object.keys(this.properties)
    if( props.length == 0 ) { return [] }

    return [`_litUpdate(${props.join(",")})`]
  }

  static get template() {
    return `
      <div id="root"></div>
    `;
  }

  connectedCallback() {
    super.connectedCallback();
    const props = Object.keys(this.constructor.properties);
    if( props.length != 0 ) {
      this._createMethodObserver(this.constructor.observers[0], true);
    }
    this._litUpdate()
  }

  _litUpdate() {
    litRender(this.render(), this.$.root);
  }
}