Tihomir, we currently fixed it like this:

const IGX_DROPDOWN_BASE = (IgxDropDownItemBaseDirective as any).ctorParameters()[0].decorators[0]
  .args[0] as InjectionToken;
  
@Component({
........
  providers: [
    {
      provide: IGX_DROPDOWN_BASE,
      useExisting: DropdownComponent,
    },
  ],
})
.......

This is really ugly but it works.
Hope you will make the token public soon.

P.S. the solution you provided won't work because the token will just have different reference.