addChild(child) {
        if (isArray(this.children) || !isArrayLike(this.children)) {
            this.children = new TreeNodes(this._tree);
            this.children._context = this;
        }

        return this.children.addNode(child);
    }
Add a child to this node.

@param {object} child Node object.
@return {TreeNode} Node object.