GeneralizingAstVisitor.visitNamedType does not call `visitTypeAnnotation`

Hello!

Currently the implementation of `GeneralizingAstVisitor.visitNamedType` is:

```dart
@override
R? visitNamedType(NamedType node) => visitNode(node);
```

The thing is, `NamedType` is a subclass of `TypeAnnotation`. It would likely be more correct to have:

```dart
@override
R? visitNamedType(NamedType node) => visitTypeAnnotation(node);
```