code: function cleanOldContentNode(el) {
          var oldContentNode = el.lightDOM,
              lightFrag = document.createDocumentFragment();
          while (oldContentNode.childNodes.length > 0) {
              lightFrag.appendChild(oldContentNode.removeChild(oldContentNode.childNodes[0]));
          }
          return lightFrag;
      }
docs: Remove and return the children of the light DOM node.
  @param {!HTMLElement} el the custom element
  @returns {DocumentFragment} the light DOM fragment