add autofocus to fix a11y issue with dialog (flutter#152637)

Add autofocus=true to first TextButton in dialog so that focus automatically goes to an interactive element when dialog is opened.

Before: https://screencast.googleplex.com/cast/NTYxNTczMTk2MDk3MTI2NHxlMjAyOTMzZi1lNw
After: https://screencast.googleplex.com/cast/NTk1NzMxNjYxNTYwMjE3NnxlYWNlM2Q1MC1jYw

fixes b/338656477

NOTE: This would be a good candidate to update the documentation for Dialogs and TextButtons to encourage the user to add autofocus=true on at least one button so that focus automatically goes to an interactive element instead of the actual Dialog element.

Author: DBowen33

dev/a11y_assessments/lib/use_cases/dialog.dart

@@ -42,6 +42,8 @@ class _MainWidget extends StatelessWidget {
                     Row(
                       children: <Widget>[
                         TextButton(
+                          key: const Key('OK Button'),
+                          autofocus: true,
                           onPressed: () {
                             Navigator.pop(context);
                           },

dev/a11y_assessments/test/dialog_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:a11y_assessments/use_cases/dialog.dart';
+import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 import 'test_utils.dart';
@@ -29,6 +30,19 @@ void main() {
     expect(find.text('This is a typical dialog.'), findsNothing);
   });
 
+  testWidgets('ok button has autofocus when dialog opens', (WidgetTester tester) async {
+    await pumpsUseCase(tester, DialogUseCase());
+
+    Future<void> invokeDialog() async {
+      await tester.tap(find.text('Show Dialog'));
+      await tester.pumpAndSettle();
+    }
+
+    await invokeDialog();
+    final Finder okButton = find.byKey(const Key('OK Button'));
+    expect((okButton.evaluate().single.widget as TextButton).autofocus, true);
+  });
+
   testWidgets('dialog has one h1 tag', (WidgetTester tester) async {
     await pumpsUseCase(tester, DialogUseCase());
     final Finder findHeadingLevelOnes = find.bySemanticsLabel('Dialog Demo');