[flutter_tools] Ensure service worker starts caching assets since first load (#116833)

* [Flutter_tools] Ensure service worker starts caching assets since the first launch

* [flutter_tools] add comment to give some context on self.clients.claim()

* [flutter_tools] add comment to give some context on self.clients.claim()

dev/bots/service_worker_test.dart

@@ -382,9 +382,8 @@ Future<void> runWebServiceWorkerTest({
       'index.html': 2,
       if (shouldExpectFlutterJs)
         'flutter.js': 1,
-      // We still download some resources multiple times if the server is non-caching.
-      'main.dart.js': 2,
-      'assets/FontManifest.json': 2,
+      'main.dart.js': 1,
+      'assets/FontManifest.json': 1,
       'flutter_service_worker.js': 1,
       'assets/AssetManifest.json': 1,
       'assets/fonts/MaterialIcons-Regular.otf': 1,
@@ -413,7 +412,6 @@ Future<void> runWebServiceWorkerTest({
       if (shouldExpectFlutterJs)
         'flutter.js': 1,
       'flutter_service_worker.js': 1,
-      'assets/fonts/MaterialIcons-Regular.otf': 1,
       'CLOSE': 1,
       if (!headless)
         'manifest.json': 1,
@@ -439,10 +437,9 @@ Future<void> runWebServiceWorkerTest({
       if (shouldExpectFlutterJs)
         'flutter.js': 1,
       'flutter_service_worker.js': 2,
-      'main.dart.js': 2,
+      'main.dart.js': 1,
       'assets/AssetManifest.json': 1,
-      'assets/FontManifest.json': 2,
-      'assets/fonts/MaterialIcons-Regular.otf': 1,
+      'assets/FontManifest.json': 1,
       'CLOSE': 1,
       if (!headless)
         ...<String, int>{

packages/flutter_tools/lib/src/web/file_generators/flutter_service_worker_js.dart

@@ -70,6 +70,8 @@ self.addEventListener("activate", function(event) {
         await caches.delete(TEMP);
         // Save the manifest to make future upgrades efficient.
         await manifestCache.put('manifest', new Response(JSON.stringify(RESOURCES)));
+        // Claim client to enable caching on first launch
+        self.clients.claim();
         return;
       }
       var oldManifest = await manifest.json();
@@ -95,6 +97,8 @@ self.addEventListener("activate", function(event) {
       await caches.delete(TEMP);
       // Save the manifest to make future upgrades efficient.
       await manifestCache.put('manifest', new Response(JSON.stringify(RESOURCES)));
+      // Claim client to enable caching on first launch
+      self.clients.claim();
       return;
     } catch (err) {
       // On an unhandled exception the state of the cache cannot be guaranteed.