_generateStaticStyle(info, rules, shadowroot, placeholder, cssBuild, cssText) {
    cssText = StyleTransformer.elementStyles(info, rules, null, cssBuild, cssText);
    if (cssText.length) {
      return StyleUtil.applyCss(cssText, info.is, shadowroot, placeholder);
    }
    return null;
  }
@param {!{is: string, extends: (string|undefined)}} info
@param {!StyleNode} rules
@param {DocumentFragment} shadowroot
@param {Node} placeholder
@param {string} cssBuild
@param {string=} cssText
@return {?HTMLStyleElement}