Method createImpl_

src/custom-element.js:1579–1586

* @return {!Promise<!./base-element.BaseElement>} * @private

()

     * @private
     */
    createImpl_() {
      return this.signals_
        .whenSignal(CommonSignals_Enum.READY_TO_UPGRADE)
        .then(() => {
          this.tryUpgrade_();
          return this.whenUpgraded();
        });
    }

    /**
     * Returns the object which holds the API surface (the thing we add the