Shadowing the type parameter in non-static nested classes

Say we have a nested class like:

```java
public class A {
public class B {
// ...
}
}
```

As far as I know, we cannot use the type parameter `T (outer)` inside `B` because `T (inner)` shadows it. So we can safely ignore the outer `T`.

- [ ] TODO: verify the above.

However we generate the code for this wrong in the unnesting process:

```dart
class B<$T extends JObject, $T extends JObject> { /* ... */ }
```
We should only generate one of the type parameters, but also correctly differentiate between the two when inferring types.

Start by tracing the unnesting process and the type-inference logic for nested generic classes. Verify the Java shadowing behavior in the issue's A/B example, then confirm that generated B declares only the applicable type parameter while inference distinguishes outer and inner parameters.

Written by the indexing model from the issue text.