code: function(storeName, transaction, cb) {
      try {
        return transaction.objectStore(storeName);
      } catch (err) {
        if (isFunction(cb)) return cb(err);
      }
    }
docs: Wrap IDBTransation.objectStore
  
  @param {String} storeName - name of the object store to use.
  @param {IDBTransaction} transaction - to use.
  @param {Function} callback - gets one argument `(err)`,
  if there is no error, it won't be called.