`unnecessary_parenthesis` not detected around literals and simple identifiers

These unnecessary parentheses should be dropped, but they are not reported:

```dart
void f() {
  final a1 = '' + ('');
  final a2 = (a1.length) * 2;
  final a3 = ((a1.isEmpty), 2);
  final a4 = (1, (2));
}
```
