tighten_type_of_initializing_formals

Use a type annotation rather than 'assert' to enforce non-nullability.

The analyzer produces this diagnostic when an assert is being used in the initializer list of a constructor to ensure that only a non-null value is being used to initialize a field.

The following code produces this diagnostic because an assert is being used to catch an error that could be caught by the type system:

class C {
  final String? s;
​
  C(this.s) : assert(s != null);
}

Remove the assert and add the non-nullable type before the initializing formal:

class C {
  final String? s;
​
  C(String this.s);
}