[jnigen] Sync member nullability throughout the class hierarchy

[HosseinYousefi]

When Foo extends Bar, then a method bar from Bar can be overriden in Foo with a potentially tighter return type (so if the original was nullable, this one can be non-nullable). The parameters of the method can be potentially looser (so if the original was non-nullable, this one can be nullable).

This also means the nullability data should be exported in the lockfile because when used for importing another package we'll use this data to determine the nullability of the subclasses.

[dcharkes]

From offline discussion

• To-non-nullable indicator for type parameters like T! language#4160 use extension types to extract the non-null type.
• If we don't want to add an extension next to the class (and the type class), we could potentially have a final field with a function type that infers the type from the extension type.

[HosseinYousefi]

we could potentially have a final field with a function type that infers the type from the extension type.

That's not possible. We don't have access to this when trying to initialize the final field.

[dcharkes]

we could potentially have a final field with a function type that infers the type from the extension type.

That's not possible. We don't have access to this when trying to initialize the final field.

late final?

[HosseinYousefi]

we could potentially have a final field with a function type that infers the type from the extension type.

That's not possible. We don't have access to this when trying to initialize the final field.

late final?

That doesn't work either:

class SomeCollection<T> {
  final List<T> _data;

  SomeCollection(this._data);

  T firstWhere(bool Function(T) fn) {
    return _data.firstWhere(fn);
  }

  late final firstNonNullField = getFirstNonNull;
}

extension SomeCollectionExt<T> on SomeCollection<T?> {
  T getFirstNonNull() => firstWhere((element) => element != null)!;
}

void main() {
  final c = SomeCollection([null, 1, 2, 3]);
  int a = c
      .firstNonNullField(); // A value of type 'int?' can't be assigned to a variable of type 'int' because 'int?' is nullable and 'int' isn't.
  int b = c.getFirstNonNull(); // Fine to use the extension method
}

And even if it did work, it would only work for methods and not getters.