migrateFromChatId property
Optional. The supergroup has been migrated from a group with the specified identifier. This number may have more than 32 significant bits and some programming languages may have difficulty/silent defects in interpreting it. But it has at most 52 significant bits, so a signed 64-bit integer or double-precision float type are safe for storing this identifier.
Implementation
final int? migrateFromChatId;