message: refactor(Subscriber): flag sync properties as internal
diff: @@ -39,9 +39,9 @@ export class Subscriber<T> extends Subscription implements Observer<T> {
  return subscriber;
  }
  - public syncErrorValue: any = null;
  - public syncErrorThrown: boolean = false;
  - public syncErrorThrowable: boolean = false;
  + /** @internal */ syncErrorValue: any = null;
  + /** @internal */ syncErrorThrown: boolean = false;
  + /** @internal */ syncErrorThrowable: boolean = false;
  protected isStopped: boolean = false;
  protected destination: PartialObserver<any>; // this `any` is the escape hatch to erase extra type param (e.g. R)
  