> The Dart compiler knows if I do a null-check, it can treat following references as safe, instead of if let x = x etc.

Does Dart bind a new variable 'x' for the if-statement scope or does the if-statement scope refer to the same 'x' from the outer scope?

The reason Swift and other languages bind a new 'x' variable (if let x = x) is because the original 'x' can be assigned a different value elsewhere, like in another thread or a closure. If a closure captures and assigns null to 'x' and there is only one binding of 'x', then that means the 'x' within the if-statement will become null _post null-check_ which is type unsound. I believe TypeScript has this "problem" as well.