[tool][web] Makes flutter.js more G3 friendly. (#120504)

* Allow any JS file in flutter loader.

* Nag only if service worker API is completely unavailable.

* Add info about Secure Contexts if that may be the reason why serviceworker is not available.

* Update sanity test.

* If service worker settings are null, do not even check if the API is available.

packages/flutter_tools/lib/src/web/file_generators/js/flutter.js

@@ -72,8 +72,7 @@ _flutter.loader = null;
      */
     constructor(validPatterns, policyName = "flutter-js") {
       const patterns = validPatterns || [
-        /\.dart\.js$/,
-        /^flutter_service_worker.js$/
+        /\.js$/,
       ];
       if (window.trustedTypes) {
         this.policy = trustedTypes.createPolicy(policyName, {
@@ -116,10 +115,19 @@ _flutter.loader = null;
      * @returns {Promise} that resolves when the latest serviceWorker is ready.
      */
     loadServiceWorker(settings) {
-      if (!("serviceWorker" in navigator) || settings == null) {
+      if (settings == null) {
         // In the future, settings = null -> uninstall service worker?
+        console.debug("Null serviceWorker configuration. Skipping.");
+        return Promise.resolve();
+      }
+      if (!("serviceWorker" in navigator)) {
+        let errorMessage = "Service Worker API unavailable.";
+        if (!window.isSecureContext) {
+          errorMessage += "\nThe current context is NOT secure."
+          errorMessage += "\nRead more: https://developer.mozilla.org/en-US/docs/Web/Security/Secure_Contexts";
+        }
         return Promise.reject(
-          new Error("Service worker not supported (or configured).")
+          new Error(errorMessage)
         );
       }
       const {

packages/flutter_tools/test/general.shard/build_system/targets/web_test.dart

@@ -881,6 +881,8 @@ void main() {
         flutter_js.generateFlutterJsFile(fileGeneratorsPath);
     expect(flutterJsContents, contains('"use strict";'));
     expect(flutterJsContents, contains('main.dart.js'));
+    expect(flutterJsContents, contains('if (!("serviceWorker" in navigator))'));
+    expect(flutterJsContents, contains(r'/\.js$/,'));
     expect(flutterJsContents, contains('flutter_service_worker.js?v='));
     expect(flutterJsContents, contains('document.createElement("script")'));
     expect(flutterJsContents, contains('"application/javascript"'));