transactionStore method Null safety

Transaction transactionStore(
  1. String storeName,
  2. String mode


Transaction transactionStore(String storeName, String mode) {
  if (mode != 'readonly' && mode != 'readwrite') {
    throw new ArgumentError(mode);
  // Try and create a transaction with a string mode.  Browsers that expect a
  // numeric mode tend to convert the string into a number.  This fails
  // silently, resulting in zero ('readonly').
  return _transaction(storeName, mode);