Dart Extension Types — Zero-Cost Wrappers and Type-Safe Domain Modeling Introduced in Dart 3.3, Extension Types let you wrap an existing type with zero runtime overhead while gaining domain-specific type safety. More powerful than typedef , cheaper than class . What Are Extension Types? An Extension Type is identical to its wrapped type at runtime, but treated as a distinct type by the static type checker. // ❌ Type-unsafe: wrong argument order compiles fine void transferMoney ( int from , int to , int amount ) { ... } transferMoney ( userId , accountId , 1000 ); // silently wrong // ✅ Extension Types prevent this at compile time extension type UserId ( int value ) {} extension type AccountId ( int value ) {} extension type Money ( int cents ) {} void transferMoney ( UserId from , AccountId to , Money amount ) { ...…