Fix offline resource loader for serviceworker (#73416)

packages/flutter_tools/lib/src/build_system/targets/web.dart

@@ -616,7 +616,7 @@ async function downloadOffline() {
     }
     currentContent[key] = true;
   }
-  for (var resourceKey in Object.keys(RESOURCES)) {
+  for (var resourceKey of Object.keys(RESOURCES)) {
     if (!currentContent[resourceKey]) {
       resources.push(resourceKey);
     }