Use service worker version template token

web/index.html

@@ -43,7 +43,6 @@
        application. For more information, see:
        https://developers.google.com/web/fundamentals/primers/service-workers -->
   <script>
-    var serviceWorkerVersion = null;
     var scriptLoaded = false;
     function loadMainDartJs() {
       if (scriptLoaded) {
@@ -62,7 +61,7 @@
         // Wait for registration to finish before dropping the <script> tag.
         // Otherwise, the browser will load the script multiple times,
         // potentially different versions.
-        var serviceWorkerUrl = 'flutter_service_worker.js?v=' + serviceWorkerVersion;
+        var serviceWorkerUrl = 'flutter_service_worker.js?v=' + {{flutter_service_worker_version}};
         navigator.serviceWorker.register(serviceWorkerUrl)
           .then((reg) => {
             function waitForActivation(serviceWorker) {
@@ -77,7 +76,7 @@
               // No active web worker and we have installed or are installing
               // one for the first time. Simply wait for it to activate.
               waitForActivation(reg.installing || reg.waiting);
-            } else if (!reg.active.scriptURL.endsWith(serviceWorkerVersion)) {
+            } else if (!reg.active.scriptURL.endsWith({{flutter_service_worker_version}})) {
               // When the app updates the serviceWorkerVersion changes, so we
               // need to ask the service worker to update.
               console.log('New service worker available.');