dart2js: toString() breaks for the JS Symbol type

Reproduction:

```dart
import 'package:js/js.dart';

@JS("Symbol")
class JSSymbol {}

@JS("Symbol")
external JSSymbol createJSSymbol(String name);

void main() {
  createJSSymbol("foo").toString();
}
```

Compiling to JS with dart2js (Dart 3.0.7) and running in the browser throws `method not found: 'toString$0' (J.getInterceptor$(...).toString$0 is not a function)`.