exists method
Implementation
@override
Future<bool> exists(String key) async {
final database = await _database;
final transaction = database.transaction(_recordStore, idbModeReadOnly);
final value = await transaction
.objectStore(_recordStore)
.getObject(_recordId(key));
await transaction.completed;
if (value != null) return true;
if (namespace != 'default' || !_hasLegacyStore) return false;
final legacy = database.transaction(_legacyStore, idbModeReadOnly);
final legacyValue = await legacy.objectStore(_legacyStore).getObject(key);
await legacy.completed;
return legacyValue != null;
}