[flutter_tools] fix the event attribute used to skip waiting (#59568)

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

@@ -523,7 +523,7 @@ self.addEventListener("fetch", (event) => {
 self.addEventListener('message', (event) => {
   // SkipWaiting can be used to immediately activate a waiting service worker.
   // This will also require a page refresh triggered by the main worker.
-  if (event.message == 'skipWaiting') {
+  if (event.data == 'skipWaiting') {
     return self.skipWaiting();
   }