transformCssText(cssText, rule) {
    // produce variables
    cssText = cssText.replace(VAR_ASSIGN, (matchText, propertyName, valueProperty, valueMixin) =>
      this._produceCssProperties(matchText, propertyName, valueProperty, valueMixin, rule));
    // consume mixins
    return this._consumeCssProperties(cssText, rule);
  }
@param {string} cssText
@param {!StyleNode} rule
@return {string}