Method markUnresolved

src/custom-element.js:389–393

* When the document is ready (meaning all external resources are loaded or * failed), we mark any stubbed elements as unresolved. If they haven't * been upgraded yet (or pending upgrade or deferredBuild elements), then * the extension failed to load.

()

     * the extension failed to load.
     */
    markUnresolved() {
      if (!this.implClass_) {
        this.classList.add('amp-unresolved', 'i-amphtml-unresolved');
      }
    }

    /**
     * Time delay imposed by baseElement upgradeCallback.  If no