fix simple dialog introducing additional node for semantics label (#68804)

* fix simple dialog introducing additional node for semantics label

* add test

packages/flutter/lib/src/material/dialog.dart

@@ -841,7 +841,7 @@ class SimpleDialog extends StatelessWidget {
         ),
         child: DefaultTextStyle(
           style: titleTextStyle ?? DialogTheme.of(context).titleTextStyle ?? theme.textTheme.headline6!,
-          child: Semantics(namesRoute: true, child: title),
+          child: Semantics(namesRoute: label == null, child: title),
         ),
       );
     }
@@ -879,6 +879,8 @@ class SimpleDialog extends StatelessWidget {
 
     if (label != null)
       dialogChild = Semantics(
+        scopesRoute: true,
+        explicitChildNodes: true,
         namesRoute: true,
         label: label,
         child: dialogChild,

packages/flutter/test/material/dialog_test.dart

@@ -1377,6 +1377,48 @@ void main() {
     semantics.dispose();
   });
 
+  testWidgets('SimpleDialog does not introduce additional node', (WidgetTester tester) async {
+    final SemanticsTester semantics = SemanticsTester(tester);
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(platform: TargetPlatform.android),
+        home: Material(
+          child: Builder(
+            builder: (BuildContext context) {
+              return Center(
+                child: ElevatedButton(
+                  child: const Text('X'),
+                  onPressed: () {
+                    showDialog<void>(
+                      context: context,
+                      builder: (BuildContext context) {
+                        return const SimpleDialog(
+                          title: Text('Title'),
+                          semanticLabel: 'label',
+                        );
+                      },
+                    );
+                  },
+                ),
+              );
+            },
+          ),
+        ),
+      ),
+    );
+
+    await tester.tap(find.text('X'));
+    await tester.pumpAndSettle();
+    // A scope route is not focusable in accessibility service.
+    expect(semantics, includesNodeWith(
+      label: 'label',
+      flags: <SemanticsFlag>[SemanticsFlag.namesRoute, SemanticsFlag.scopesRoute],
+    ));
+
+    semantics.dispose();
+  });
+
   testWidgets('Dismissible.confirmDismiss defers to an AlertDialog', (WidgetTester tester) async {
     final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
     final List<int> dismissedItems = <int>[];