code: function stat(dir, files, cb) {
    var batch = new Batch();
  
    batch.concurrency(10);
  
    files.forEach(function(file){
      batch.push(function(done){
        fs.stat(join(dir, file), function(err, stat){
          if (err && err.code !== 'ENOENT') return done(err);
  
          // pass ENOENT as null stat, not error
          done(null, stat || null);
        });
      });
    });
  
    batch.end(cb);
  }
docs: Stat all files and return array of stat
  in same order.