code: function lookup( name, ext, paths ) {
      var files = [],
          filename = name,
          basename = filename,
          dirname = '',
          mapping,
          i, j, len, len2, path, info;
  
      files.push( filename );
  
      if ( /^(.*(?:\/|\\))([^\/\\]*?)$/.exec( filename ) ) {
          dirname = RegExp.$1;
          basename = RegExp.$2;
      }
  
      // 自动加后缀。
      if ( !/\.(scss|sass)$/.exec( basename ) ) {
          filename = dirname + basename + ext;
          files.push( filename );
  
          if ( !/^_/.exec( basename ) ) {
              filename = dirname + '_' + basename + ext;
              files.push( filename );
          }
  
          mapping = {
              '.sass': '.scss',
              '.scss': '.sass'
          };
  
          if ( mapping[ ext ] ) {
              filename = dirname + basename + mapping[ ext ];
              files.push( filename );
  
              if ( !/^_/.exec( basename ) ) {
                  filename = dirname + '_' + basename + mapping[ ext ];
                  files.push( filename );
              }
          }
      }
  
      len = files.length;
      len2 = paths.length;
  
      for ( i = 0; i < len; i++ ) {
          name = files[ i ];
  
          for ( j = 0; j < len2; j++ ) {
              info = fis.uri( name, paths[ j ] );
  
              if( info.file && info.file.isFile() ) {
                  return info.file;
              }
          }
      }
  
      // console.log( paths, files );
  }
docs: 查找文件。