Explain sound null safety in Dart. How does it differ from Kotlin's null safety?

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Dart sound null safety (since Dart 2.12 / Flutter 2.0): types are non-nullable by default ; ? opts in to nullability; the compiler proves no null dereferences at compile time. Key operators: ? nullable type, ! force-unwrap, ?. null-safe access, ?? null-coalesce, ??= assign-if-null, late deferred init, required for named params. Dart vs Kotlin: Both sound (compiler-proven safety). Dart's late works on final fields; Kotlin's lateinit doesn't. Dart has an explicit required keyword for named parameters; Kotlin uses non-null defaults + optional params. Force-unwrap: ! in Dart, !! in Kotlin. Null-coalesce: ?? in Dart, ?: 'Elvis' operator in Kotlin. Opting out of null safety is no longer supported in modern Flutter. Two terms to use precisely: **'sound null safety'** (the compiler proves no null dereferences — Dart 2.12+) and **non-nullable by default**. The Kotlin vs Dart comparison interviewers like: (1) **`late` in Dart works on `final` fields, Kotlin's `lateinit` doesn't**; (2) **Dart has `required` for named parameters explicitly** — Kotlin handles it via non-null defaults. End with the practical signal: 'in modern Flutter (since 2.0/Dart 2.12), opting out of null safety is no longer supported'.