Wait until all scripts are loaded in the page before running main for the DDC library bundle format (#162707)

flutter/flutter#162567

- Uses the `bootstrapScript` field in `loadConfig` to run a script after
all scripts have loaded.
- This script just calls a callback that is set up beforehand and calls
main.
- Modifies the callback that calls `dartDevEmbedder.runMain` to wait
until both DWDS called main and all scripts have loaded.
- Unskips hot reload tests now that the race condition should no longer
exist.

## Pre-launch Checklist

- [ ] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [ ] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [ ] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [ ] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

Author: srujzs

packages/flutter_tools/lib/src/isolated/devfs_web.dart

@@ -1087,13 +1087,18 @@ class WebDevFS implements DevFS {
               generateLoadingIndicator: enableDwds,
             ),
       );
+      const String onLoadEndBootstrap = 'on_load_end_bootstrap.js';
+      if (ddcModuleSystem) {
+        webAssetServer.writeFile(onLoadEndBootstrap, generateDDCLibraryBundleOnLoadEndBootstrap());
+      }
       webAssetServer.writeFile(
         'main_module.bootstrap.js',
         ddcModuleSystem
             ? generateDDCLibraryBundleMainModule(
               entrypoint: entrypoint,
               nullAssertions: nullAssertions,
               nativeNullAssertions: nativeNullAssertions,
+              onLoadEndBootstrap: onLoadEndBootstrap,
             )
             : generateMainModule(
               entrypoint: entrypoint,

packages/flutter_tools/lib/src/web/bootstrap.dart

@@ -505,10 +505,13 @@ String generateDDCMainModule({
 ''';
 }
 
+const String _onLoadEndCallback = r'$onLoadEndCallback';
+
 String generateDDCLibraryBundleMainModule({
   required String entrypoint,
   required bool nullAssertions,
   required bool nativeNullAssertions,
+  required String onLoadEndBootstrap,
 }) {
   // The typo below in "EXTENTION" is load-bearing, package:build depends on it.
   return '''
@@ -519,21 +522,48 @@ String generateDDCLibraryBundleMainModule({
 
   dartDevEmbedder.debugger.registerDevtoolsFormatter();
 
-  let child = {};
-  child.main = function() {
-    let sdkOptions = {
-      nonNullAsserts: $nullAssertions,
-      nativeNonNullAsserts: $nativeNullAssertions,
-    };
-    dartDevEmbedder.runMain(appName, sdkOptions);
+  // Set up a final script that lets us know when all scripts have been loaded.
+  let onLoadEndSrc = '$onLoadEndBootstrap';
+  window.\$dartLoader.loadConfig.bootstrapScript = {
+    src: onLoadEndSrc,
+    id: onLoadEndSrc,
+  };
+  window.\$dartLoader.loadConfig.tryLoadBootstrapScript = true;
+  let dwdsCalledMain = false;
+  let dartSrcsLoaded = false;
+  let runMainWhenBoth = function() {
+    // Only run once both all the scripts are loaded and DWDS triggers main.
+    if (dwdsCalledMain && dartSrcsLoaded) {
+      let sdkOptions = {
+        nonNullAsserts: $nullAssertions,
+        nativeNonNullAsserts: $nativeNullAssertions,
+      };
+      dartDevEmbedder.runMain(appName, sdkOptions);
+    }
+  }
+  // DWDS expects the main function to be lowercase.
+  // TODO(srujzs): DWDS should be more robust to not have to require that.
+  dwdsmain = function() {
+    dwdsCalledMain = true;
+    runMainWhenBoth();
+  }
+  // Should be called by $onLoadEndBootstrap once all the scripts have been
+  // loaded.
+  window.$_onLoadEndCallback = function() {
+    dartSrcsLoaded = true;
+    runMainWhenBoth();
   }
 
   /* MAIN_EXTENSION_MARKER */
-  child.main();
+  dwdsmain();
 })();
 ''';
 }
 
+String generateDDCLibraryBundleOnLoadEndBootstrap() {
+  return '''window.$_onLoadEndCallback();''';
+}
+
 /// Generate a synthetic main module which captures the application's main
 /// method.
 ///

packages/flutter_tools/test/general.shard/web/bootstrap_test.dart

@@ -273,6 +273,7 @@ void main() {
         entrypoint: 'main.js',
         nullAssertions: false,
         nativeNullAssertions: false,
+        onLoadEndBootstrap: 'on_load_end_bootstrap.js',
       );
       // bootstrap main module has correct defined module.
       expect(result, contains('let appName = "org-dartlang-app:/main.js";'));
@@ -284,6 +285,7 @@ void main() {
         entrypoint: 'main.js',
         nullAssertions: true,
         nativeNullAssertions: true,
+        onLoadEndBootstrap: 'on_load_end_bootstrap.js',
       );
 
       expect(result, contains('nonNullAsserts: true'));
@@ -295,6 +297,7 @@ void main() {
         entrypoint: 'main.js',
         nullAssertions: false,
         nativeNullAssertions: false,
+        onLoadEndBootstrap: 'on_load_end_bootstrap.js',
       );
 
       expect(result, contains('nonNullAsserts: false'));

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

@@ -12,11 +12,7 @@ import '../test_driver.dart';
 import '../test_utils.dart';
 import 'hot_reload_project.dart';
 
-void testAll({
-  bool chrome = false,
-  List<String> additionalCommandArgs = const <String>[],
-  Object? skip = false,
-}) {
+void testAll({bool chrome = false, List<String> additionalCommandArgs = const <String>[]}) {
   group('chrome: $chrome'
       '${additionalCommandArgs.isEmpty ? '' : ' with args: $additionalCommandArgs'}', () {
     late Directory tempDir;
@@ -235,7 +231,7 @@ void testAll({
       // isolates, so this test will wait forever.
       skip: chrome,
     );
-  }, skip: skip);
+  });
 }
 
 bool _isHotReloadCompletionEvent(Map<String, Object?>? event) {

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

@@ -11,11 +11,7 @@ import '../test_driver.dart';
 import '../test_utils.dart';
 import 'hot_reload_with_asset.dart';
 
-void testAll({
-  bool chrome = false,
-  List<String> additionalCommandArgs = const <String>[],
-  Object? skip = false,
-}) {
+void testAll({bool chrome = false, List<String> additionalCommandArgs = const <String>[]}) {
   group('chrome: $chrome'
       '${additionalCommandArgs.isEmpty ? '' : ' with args: $additionalCommandArgs'}', () {
     late Directory tempDir;
@@ -86,5 +82,5 @@ void testAll({
       await flutter.hotRestart();
       await onSecondLoad.future;
     });
-  }, skip: skip);
+  });
 }

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

@@ -13,11 +13,7 @@ import '../test_utils.dart';
 
 // This test verifies that we can hot reload a stateless widget into a
 // stateful one and back.
-void testAll({
-  bool chrome = false,
-  List<String> additionalCommandArgs = const <String>[],
-  Object? skip = false,
-}) {
+void testAll({bool chrome = false, List<String> additionalCommandArgs = const <String>[]}) {
   group('chrome: $chrome'
       '${additionalCommandArgs.isEmpty ? '' : ' with args: $additionalCommandArgs'}', () {
     late Directory tempDir;
@@ -60,5 +56,5 @@ void testAll({
       expect(logs, contains('STATEFUL'));
       await subscription.cancel();
     });
-  }, skip: skip);
+  });
 }

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

@@ -5,8 +5,6 @@
 @Tags(<String>['flutter-test-driver'])
 library;
 
-import 'dart:io';
-
 import '../integration.shard/test_data/hot_reload_errors_common.dart';
 import '../src/common.dart';
 
@@ -19,7 +17,5 @@ void main() {
     // TODO(srujzs): Remove this custom message once we have the delta inspector emitting the same
     // string as the VM.
     constClassFieldRemovalErrorMessage: 'Const class cannot remove fields',
-    // https://github.com/flutter/flutter/issues/162567
-    skip: Platform.isWindows,
   );
 }

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

@@ -14,7 +14,5 @@ void main() {
     additionalCommandArgs: <String>[
       '--extra-front-end-options=--dartdevc-canary,--dartdevc-module-format=ddc',
     ],
-    // https://github.com/flutter/flutter/issues/162567
-    skip: true,
   );
 }

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

@@ -14,7 +14,5 @@ void main() {
     additionalCommandArgs: <String>[
       '--extra-front-end-options=--dartdevc-canary,--dartdevc-module-format=ddc',
     ],
-    // https://github.com/flutter/flutter/issues/162567
-    skip: true,
   );
 }

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

@@ -14,7 +14,5 @@ void main() {
     additionalCommandArgs: <String>[
       '--extra-front-end-options=--dartdevc-canary,--dartdevc-module-format=ddc',
     ],
-    // https://github.com/flutter/flutter/issues/162567
-    skip: true,
   );
 }