remove_(storeName, keys, onRemove) {
    const op = this.connection_.startReadWriteOperation(storeName);
    const store = op.store();

    for (const key of keys) {
      store.delete(key).onsuccess = () => onRemove(key);
    }

    return op.promise();
  }
@param {string} storeName
@param {!Array<number>} keys
@param {function(number)} onRemove
@return {!Promise}
@private