Allow `@JSExport()` annotation on generated mocks

[srujzs]

With dart:js_interop, mocking needs to be done at the JS level due to the use of extension types.

For example:

extension type Window._(JSObject _) implements JSObject {
  external String get name;
  void method() {}
}

name will call the member within the underlying JS object, whereas method is unmockable (due to static dispatch).

To help create mocks for this as well as to export Dart objects, we added @JSExport() and createJSInteropWrapper e.g.

@JSExport()
class WindowFake {
  String get name => 'fake';
}

void main() {
  var wrapper = createJSInteropWrapper(WindowFake()) as Window;
  wrapper.name; // 'fake'
}

This creates a JS wrapper around the Dart object and forwards the JS interop calls to the Dart object. @JSExport is needed in order to do some validation and identify a class as "exportable".

GenerateNiceMocks allows users to generate a mock given a definition like WindowFake. This allows users to stub calls instead of just using a fake, but it requires the generated mock to have a @JSExport annotation, something which we don't have support for today.

[srujzs]

It might even be better if we expose another method like GenerateNiceMocks that generates the needed class e.g. WindowFake given an extension type. That makes it so the user has to do little beyond calling something like GenerateNiceJsInteropMocks([MockSpec<Window>()], which will emit a mockable class MockWindow all by itself.