Widget getIcon(String url) {
  return FutureBuilder(
    future: existSite(url),
    builder: (BuildContext context, AsyncSnapshot<bool> snapshot) { // 第二引数はfutureの戻りの型
      if (snapshot.hasData && snapshot.data) {
        return CachedNetworkImage(
          imageUrl: 'http://favicon.st-hatena.com/?url=$url',
          placeholder: (context, url) => Icon(Icons.not_interested),
        );
      } else {
        return Icon(Icons.not_interested);
      }
    },
  );