[js_interop] No warning if a JS interop type constructor is annotated with `@JS()`

[sgrekhov]

extension type ET._(JSObject _) implements JSObject {
  @JS("whatever")
  external ET(int? id, String? name);

  external int? get id;
  external String? get name;
}

The @JS() annotation on the constructor above has no effect and should probably trigger a warning. @srujzs ?

[srujzs]

Agreed!