[web] define $flutterDriverResult variable early in driver test initalization (#107337)

dev/bots/test.dart

@@ -887,7 +887,11 @@ Future<void> _runFrameworkTests() async {
     await _runFlutterTest(path.join(flutterRoot, 'dev', 'tools', 'gen_keycodes'));
     await _runFlutterTest(path.join(flutterRoot, 'dev', 'benchmarks', 'test_apps', 'stocks'));
     await _runFlutterTest(path.join(flutterRoot, 'packages', 'flutter_driver'), tests: <String>[path.join('test', 'src', 'real_tests')], options: soundNullSafetyOptions);
-    await _runFlutterTest(path.join(flutterRoot, 'packages', 'integration_test'), options: <String>['--enable-vmservice']);
+    await _runFlutterTest(path.join(flutterRoot, 'packages', 'integration_test'), options: <String>[
+      '--enable-vmservice',
+      // Web-specific tests depend on Chromium, so they run as part of the web_long_running_tests shard.
+      '--exclude-tags=web',
+    ]);
     await _runFlutterTest(path.join(flutterRoot, 'packages', 'flutter_goldens'), options: soundNullSafetyOptions);
     await _runFlutterTest(path.join(flutterRoot, 'packages', 'flutter_localizations'), options: soundNullSafetyOptions);
     await _runFlutterTest(path.join(flutterRoot, 'packages', 'flutter_test'), options: soundNullSafetyOptions);
@@ -1115,6 +1119,16 @@ Future<void> _runWebLongRunningTests() async {
     () => _runWebReleaseTest('lib/sound_mode.dart', additionalArguments: <String>[
       '--sound-null-safety',
     ]),
+    () => _runFlutterWebTest(
+      'html',
+      path.join(flutterRoot, 'packages', 'integration_test'),
+      <String>['test/web_extension_test.dart'],
+    ),
+    () => _runFlutterWebTest(
+      'canvaskit',
+      path.join(flutterRoot, 'packages', 'integration_test'),
+      <String>['test/web_extension_test.dart'],
+    ),
   ];
 
   // Shuffling mixes fast tests with slow tests so shards take roughly the same

packages/flutter_driver/lib/src/driver/web_driver.dart

@@ -294,6 +294,29 @@ class FlutterWebConnection {
 
   /// Sends command via WebDriver to Flutter web application.
   Future<dynamic> sendCommand(String script, Duration? duration) async {
+    // This code should not be reachable before the VM service extension is
+    // initialized. The VM service extension is expected to initialize both
+    // `$flutterDriverResult` and `$flutterDriver` variables before attempting
+    // to send commands. This part checks that `$flutterDriverResult` is present.
+    // `$flutterDriver` is not checked because it is covered by the `script`
+    // that's executed next.
+    try {
+      await _driver.execute(r'return $flutterDriverResult', <String>[]);
+    } catch (error, stackTrace) {
+      throw DriverError(
+        'Driver extension has not been initialized correctly.\n'
+        'If the test uses a custom VM service extension, make sure it conforms '
+        'to the protocol used by package:integration_test and '
+        'package:flutter_driver.\n'
+        'If the test uses VM service extensions provided by the Flutter SDK, '
+        'then this error is likely caused by a bug in Flutter. Please report it '
+        'by filing a bug on GitHub:\n'
+        '  https://github.com/flutter/flutter/issues/new?template=2_bug.md',
+        error,
+        stackTrace,
+      );
+    }
+
     String phase = 'executing';
     try {
       // Execute the script, which should leave the result in the `$flutterDriverResult` global variable.

packages/flutter_driver/lib/src/extension/_extension_web.dart

@@ -18,6 +18,11 @@ import 'dart:js_util' as js_util;
 ///
 ///  * [_extension_io.dart], which has the dart:io implementation
 void registerWebServiceExtension(Future<Map<String, dynamic>> Function(Map<String, String>) call) {
+  // Define the result variable because packages/flutter_driver/lib/src/driver/web_driver.dart
+  // checks for this value to become non-null when waiting for the result. If this value is
+  // undefined at the time of the check, WebDriver throws an exception.
+  context[r'$flutterDriverResult'] = null;
+
   js_util.setProperty(html.window, r'$flutterDriver', allowInterop((dynamic message) async {
     final Map<String, String> params = Map<String, String>.from(
         jsonDecode(message as String) as Map<String, dynamic>);

packages/flutter_driver/test/src/web_tests/web_extension_test.dart

@@ -20,7 +20,12 @@ void main() {
     test('web_extension should register a function', () {
       expect(() => registerWebServiceExtension(call),
           returnsNormally);
+
       expect(js.context.hasProperty(r'$flutterDriver'), true);
+      expect(js.context[r'$flutterDriver'], isNotNull);
+
+      expect(js.context.hasProperty(r'$flutterDriverResult'), true);
+      expect(js.context[r'$flutterDriverResult'], isNull);
     });
   });
 }

packages/integration_test/lib/_extension_web.dart

@@ -17,6 +17,11 @@ import 'dart:js_util' as js_util;
 ///
 ///  * `_extension_io.dart`, which has the dart:io implementation
 void registerWebServiceExtension(Future<Map<String, dynamic>> Function(Map<String, String>) callback) {
+  // Define the result variable because packages/flutter_driver/lib/src/driver/web_driver.dart
+  // checks for this value to become non-null when waiting for the result. If this value is
+  // undefined at the time of the check, WebDriver throws an exception.
+  context[r'$flutterDriverResult'] = null;
+
   js_util.setProperty(html.window, r'$flutterDriver', allowInterop((dynamic message) async {
     try {
       final Map<String, dynamic> messageJson = jsonDecode(message as String) as Map<String, dynamic>;

packages/integration_test/test/web_extension_test.dart

@@ -0,0 +1,21 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+@Tags(<String>['web'])
+import 'dart:js' as js;
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:integration_test/integration_test.dart';
+
+void main() {
+  IntegrationTestWidgetsFlutterBinding();
+
+  test('IntegrationTestWidgetsFlutterBinding on the web should register certain global properties', () {
+    expect(js.context.hasProperty(r'$flutterDriver'), true);
+    expect(js.context[r'$flutterDriver'], isNotNull);
+
+    expect(js.context.hasProperty(r'$flutterDriverResult'), true);
+    expect(js.context[r'$flutterDriverResult'], isNull);
+  });
+}