`avoid_unnecessary_return_variable` triggers when type promotion is needed


```dart
T get<T>(String key) {
  final value = _map[key];
  
  if (value is T) {
    // local variable is promoted to T
    return value;
  }

  throw Exception('value is not of type $T');
}
```

### Expected behavior
No lint

### Actual behavior
The lint rule is triggered