code: function xLang(tag, content, file, defaultExt) {
    var ext = defaultExt;
  
    if (file.pipeEmbed === false) {
      switch (ext) {
        case 'html':
          content = extHtml(content, null, file);
          break;
  
        case 'js':
          content = extJs(content, null, file);
          break;
  
        case 'css':
          content = extCss(content, null, file);
          break;
      }
  
      return tag + content;
    } else {
      var isXLang = false;
  
      var m = rType.exec(tag);
      if (m) {
        var lang = m[3].toLowerCase();
  
        switch (lang) {
          case 'javascript':
            ext = 'js';
            break;
  
          case 'css':
            ext = 'css';
            break;
  
          default:
            if (lang.substring(0, 2) === 'x-') {
              ext = lang.substring(2);
              isXLang = true;
            }
            break;
        }
      }
  
      if (isXLang) {
        var mime = _.getMimeType(ext);
        mime && (mime !== 'application/x-' + ext) && (tag = tag.replace(rType, function(all, quote) {
          return ' type=' + quote + mime + quote;
        }));
      }
    }
  
    return tag + map.xlang.wrap(content, ext);
  }
docs: 处理type类型为 `x-**` 的block标签。
  
  ```css
  <head>
  <style type="x-scss">
  &commat;import "compass/css3";
  
  #border-radius {
  &commat;include border-radius(25px);
  }
  </style>
  </head>
  ```
  @param  {String} tag        标签
  @param  {String} content    the content of file
  @param  {File} file         fis.file instance
  @param  {String} defaultExt what is ?
  @return {String}
  @function
  @memberOf fis.compile