dart2js passes type parameters to @JS constructors

[harryterkelsen]

We have a @js annotated class with a type parameter:

typedef SkObjectFinalizer<T> = void Function(T key);

 @JS('window.FinalizationRegistry')
class SkObjectFinalizationRegistry<T> {
  external SkObjectFinalizationRegistry(SkObjectFinalizer<T> finalizer);
  external void register(Object ckObject, Object skObject);
}

And an instantiation:

final SkObjectFinalizationRegistry<SkDeletable> skObjectFinalizationRegistry = SkObjectFinalizationRegistry<SkDeletable>(js.allowInterop((SkDeletable deletable) {
  _skObjectDeleteQueue.add(deletable);
  _skObjectCollector ??= _scheduleSkObjectCollection();
}));

This causes an exception at runtime due to passing the SkDeletable type to the FinalizationRegistry constructor. The generated code is something like this:

new FinalizationRegistry(new H.closure(), lookupType('SkDeletable'));

I expect for dart2js not to pass type parameters to native constructors.

[sigmundch]

My first thought here would be to change the original code to remove the type parameter, seems like @hterkelsen you already have a PR to do so, correct? If so, I'm keeping this as a P2 to eventually fix dart2js.

Because generics are not reified on JS-interop types, I'm wondering whether this should be a static compile-time error instead. If we decide to go down that path, we could add it as part of the js-interop validation checks that @srujzs has been working on recently, my only fear is that this would be more breaking than other checks we've been adding so far.

[srujzs]

FYI: there are related tasks to be done in #32929 regarding specifying and adding errors for generics.