[ci] Adds test for web hot restart with const App. (#141824)

This PR adds a test that reproduces the problem described in the linked issue: hot restart on the web seems to not update if the app being run is `const`.

The new test is expected to fail, until the `const` issue with hot restart in the web is resolved.

Expected failure mode is a 15s timeout in the following test:

```
02:31 +3 ~1 -1: Hot reload (index.html: Default) (with `const MyApp()`)): newly added code executes during hot restart [E]
  TimeoutException after 0:00:15.000000: Future not completed
  dart:async  _startMicrotaskLoop
  ...
```

(And then a bunch of output that I'm not 100% sure is intended :))

## Issues

* #141588

packages/flutter_tools/test/integration.shard/test_data/hot_reload_project.dart

@@ -6,7 +6,9 @@ import '../test_utils.dart';
 import 'project.dart';
 
 class HotReloadProject extends Project {
-  HotReloadProject({super.indexHtml});
+  HotReloadProject({super.indexHtml, this.constApp = false});
+
+  final bool constApp;
 
   @override
   final String pubspec = '''
@@ -20,7 +22,7 @@ class HotReloadProject extends Project {
   ''';
 
   @override
-  final String main = r'''
+  String get main => '''
   import 'package:flutter/material.dart';
   import 'package:flutter/scheduler.dart';
   import 'package:flutter/services.dart';
@@ -31,20 +33,14 @@ class HotReloadProject extends Project {
     WidgetsFlutterBinding.ensureInitialized();
     final ByteData message = const StringCodec().encodeMessage('AppLifecycleState.resumed')!;
     await ServicesBinding.instance!.defaultBinaryMessenger.handlePlatformMessage('flutter/lifecycle', message, (_) { });
-    // See https://github.com/flutter/flutter/issues/86202
-    if (kIsWeb) {
-      while (true) {
-        runApp(MyApp());
-        await Future.delayed(const Duration(seconds: 1));
-      }
-    } else {
-     runApp(MyApp());
-    }
+    runApp(${constApp ? 'const ': ''}MyApp());
   }
 
   int count = 1;
 
   class MyApp extends StatelessWidget {
+    ${constApp ? 'const MyApp({super.key});': ''}
+
     @override
     Widget build(BuildContext context) {
       // This method gets called each time we hot reload, during reassemble.
@@ -53,7 +49,7 @@ class HotReloadProject extends Project {
       // hot reloading worked:
       // printHotReloadWorked();
 
-      print('((((TICK $count))))');
+      print('((((TICK \$count))))');
       // tick 1 = startup warmup frame
       // tick 2 = hot reload warmup reassemble frame
       // after that there's a post-hot-reload frame scheduled by the tool that
@@ -77,10 +73,20 @@ class HotReloadProject extends Project {
     }
   }
 
-  void printHotReloadWorked() {
+  Future<void> printHotReloadWorked() async {
     // The call to this function is uncommented by a test to verify that hot
     // reloading worked.
     print('(((((RELOAD WORKED)))))');
+
+    // We need to insist here for `const` Apps, so print statements don't
+    // get lost between the browser and the test driver.
+    // See: https://github.com/flutter/flutter/issues/86202
+    if (kIsWeb) {
+      while (true) {
+        await Future.delayed(const Duration(seconds: 1));
+        print('(((((RELOAD WORKED)))))');
+      }
+    }
   }
   ''';
 

packages/flutter_tools/test/web.shard/hot_reload_web_test.dart

@@ -15,6 +15,7 @@ import 'test_data/hot_reload_index_html_samples.dart';
 
 void main() async {
   await _testProject(HotReloadProject()); // default
+  await _testProject(HotReloadProject(constApp: true), name: 'Default) (with `const MyApp()`)'); // runApp(const MyApp());
   await _testProject(HotReloadProject(indexHtml: indexHtmlFlutterJsCallback), name: 'flutter.js (callback)');
   await _testProject(HotReloadProject(indexHtml: indexHtmlFlutterJsPromisesFull), name: 'flutter.js (promises)');
   await _testProject(HotReloadProject(indexHtml: indexHtmlFlutterJsPromisesShort), name: 'flutter.js (promises, short)');