use a const in the index.html

@@ -34,7 +34,7 @@
 
 
   <script>
   <script>
     // The value below is injected by flutter build, do not touch.
     // The value below is injected by flutter build, do not touch.
-    var serviceWorkerVersion = null;
+    const serviceWorkerVersion = null;
   </script>
   </script>
   <!-- This script adds the flutter initialization JS code -->
   <!-- This script adds the flutter initialization JS code -->
   <script src="flutter.js" defer></script>
   <script src="flutter.js" defer></script>