Non nullable field with Built type is not enforced

The following assertion fails (doesn't throw)

```dart
expect(
() => Foo(),
throwsA(isA()),
reason: 'Should fail without required bar param',
);
```
```dart
abstract class Foo implements Built {
Bar get bar;
}

abstract class Bar implements Built {}
```

It also fails on deserialization, when bar is null in the serialized input.

It passes, when the `Foo` is annotated with `@BuiltValue(autoCreateNestedBuilders: false)` but I'd like that as `true`. Would there be any other way?