Change RTI to handle new Interceptor class for static interop classes

[srujzs]

Static interop classes will introduce a new type that is a supertype of all web Native classes and all interop classes. While the types that correspond to these classes will be erased at compile time, they still need to be made sure that all subtyping relationships act as expected.

[jodinathan]

does this new Native supertype change what can be done in JS interop?

[srujzs]

Yep. The goal is to allow @JS classes to be used on native types (which are currently reserved for Native classes) with the requirement that the classes need to be used statically (i.e. no instance members because of dynamic invocations) and is annotated with @staticInterop. For example,

@JS()
@staticInterop
class Window {}

extension on Window {
  external dynamic get navigator;
}

can be used to interop with Window objects. The pieces are there, there's just a dependency that needs to be updated before this is shipped.