[dart2wasm] Generics unexpected runtimeType when extends to nullable

```dart
void main() {
  print(AlwaysNullableData('').runtimeType);
}

abstract class BaseData<T> {
  BaseData(this.data);

  T data;
}

class AlwaysNullableData<T> extends BaseData<T?> {
  AlwaysNullableData(super.data);
}
```
This code will print `AlwaysNullableData<String>` on other platform, but on `dart2wasm` it print `AlwaysNullableData<String?>`