switch to late final

Author: shaunpanjabi

packages/in_app_purchase/in_app_purchase_storekit/test/in_app_purchase_storekit_2_platform_test.dart

@@ -163,7 +163,7 @@ void main() {
       final Stream<List<PurchaseDetails>> stream =
           iapStoreKitPlatform.purchaseStream;
 
-      late StreamSubscription<List<PurchaseDetails>> subscription;
+      late final StreamSubscription<List<PurchaseDetails>> subscription;
       subscription = stream.listen((List<PurchaseDetails> purchaseDetailsList) {
         details.addAll(purchaseDetailsList);
         if (purchaseDetailsList.first.status == PurchaseStatus.purchased) {