addToParent() {
    var parentNode = this.parentEl = this.parentNode;

    // `!parentNode` check primarily for unit tests.
    if (!parentNode || !parentNode.add || this.attachedToParent) {
      return;
    }
    parentNode.add(this);
    this.attachedToParent = true; // To prevent multiple attachments to same parent.
  }
Tell parentNode to add this entity to itself.