Create type adapters conforming to Effective Dart style

According to the Effective Dart style guidelines, [public members should be type annotated](https://dart-lang.github.io/linter/lints/type_annotate_public_apis.html).

However, the current code doesn't do that:

```dart
class …Adapter extends TypeAdapter<…> {
@override
final typeId = …;

…
}
```
instead of
```dart
class …Adapter extends TypeAdapter<…> {
@override
final int typeId = …;

…
}
```