normalize() {
    var ret = this.visit(this.root);

    if (this.hoist) {
      // hoist @import
      if (this.imports.length) ret.nodes = this.imports.concat(ret.nodes);

      // hoist @charset
      if (this.charset) ret.nodes = [this.charset].concat(ret.nodes);
    }

    return ret;
  }
Normalize the node tree.

@return {Node}
@api private