code: function(parentNode) {
          if (parentNode && parentNode.children) {
              for (var i=0, len=parentNode.children.length; i<len ; ++i) {
                  if (parentNode.children[i] === this) {
                      return i;
                  }
              }
          }
  
          return -1;
      }
docs: Returns true if the Node is a child of supplied Node
  @method isChildOf
  @param parentNode {Node} the Node to check
  @return {boolean} The node index if this Node is a child of
  supplied Node, else -1.
  @private