Null safety and the type system

Dart's sound null safety model and the depth of its type system: nullable `T?` versus non-nullable `T`, the null-aware operators, flow analysis and why promotion does not apply to fields, how `late` works internally and lazy initialization, generics, bounded types, the runtime price of covariance, plus the `==`/`hashCode` contract and immutable value objects. These come up in Flutter interviews as questions about type safety and model design.