CalendarDatePicker doesn't announce selected date on desktop (#143583)

fixes [Screen reader is not announcing the selected date as selected on DatePicker](flutter/flutter#143439)

### Descriptions
- This fixes an issue where `CalendarDatePicker` doesn't announce selected date on desktop.
- Add semantic label to describe the selected date is indeed "Selected".

### Code sample

<details>
<summary>expand to view the code sample</summary> 

```dart
import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @OverRide
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @OverRide
  MyHomePageState createState() => MyHomePageState();
}

class MyHomePageState extends State<MyHomePage> {
  void _showDatePicker() async {
    await showDatePicker(
      context: context,
      initialDate: DateTime.now(),
      firstDate: DateTime(1900),
      lastDate: DateTime(2200),
    );
  }

  @OverRide
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title,
            style: const TextStyle(fontFamily: 'ProductSans')),
      ),
      body: const Center(
        child: Text('Click the button to show date picker.'),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _showDatePicker,
        tooltip: 'Show date picker',
        child: const Icon(Icons.edit_calendar),
      ),
    );
  }
}

// import 'package:flutter/material.dart';

// void main() => runApp(const MyApp());

// class MyApp extends StatelessWidget {
//   const MyApp({super.key});

//   @OverRide
//   Widget build(BuildContext context) {
//     return MaterialApp(
//       debugShowCheckedModeBanner: false,
//       home: Scaffold(
//         body: Center(
//           child: CalendarDatePicker(
//             initialDate: DateTime.now(),
//             firstDate: DateTime(2020),
//             lastDate: DateTime(2050),
//             onDateChanged: (date) {
//               print(date);
//             },
//           ),
//         ),
//       ),
//     );
//   }
// }
```

</details>

### Before

https://github.com/flutter/flutter/assets/48603081/c82e1f15-f067-4865-8a5a-1f3c0c8d91da

### After

https://github.com/flutter/flutter/assets/48603081/193d9e26-df9e-4d89-97ce-265c3d564607

Author: TahaTesser

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

@@ -265,6 +265,21 @@ class _CalendarDatePickerState extends State<CalendarDatePicker> {
     setState(() {
       _selectedDate = value;
       widget.onDateChanged(_selectedDate!);
+      switch (Theme.of(context).platform) {
+        case TargetPlatform.linux:
+        case TargetPlatform.macOS:
+        case TargetPlatform.windows:
+          final bool isToday = DateUtils.isSameDay(widget.currentDate, _selectedDate);
+          final String semanticLabelSuffix = isToday ? ', ${_localizations.currentDateLabel}' : '';
+          SemanticsService.announce(
+            '${_localizations.selectedDateLabel} ${_localizations.formatFullDate(_selectedDate!)}$semanticLabelSuffix',
+            _textDirection,
+          );
+        case TargetPlatform.android:
+        case TargetPlatform.iOS:
+        case TargetPlatform.fuchsia:
+          break;
+      }
     });
   }
 

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

@@ -182,6 +182,10 @@ abstract class MaterialLocalizations {
   /// Label indicating that a given date is the current date.
   String get currentDateLabel;
 
+  /// The semantics label to describe the selected date in the calendar picker
+  /// invoked using [showDatePicker].
+  String get selectedDateLabel;
+
   /// Label for the scrim rendered underneath a [BottomSheet].
   String get scrimLabel;
 
@@ -1108,6 +1112,9 @@ class DefaultMaterialLocalizations implements MaterialLocalizations {
   @override
   String get currentDateLabel => 'Today';
 
+  @override
+  String get selectedDateLabel => 'Selected';
+
   @override
   String get scrimLabel => 'Scrim';
 

packages/flutter/test/material/calendar_date_picker_test.dart

@@ -1086,6 +1086,51 @@ void main() {
         }
         semantics.dispose();
       });
+
+      // This is a regression test for https://github.com/flutter/flutter/issues/143439.
+      testWidgets('Selected date Semantics announcement on onDateChanged', (WidgetTester tester) async {
+        final SemanticsHandle semantics = tester.ensureSemantics();
+        const DefaultMaterialLocalizations localizations = DefaultMaterialLocalizations();
+        final DateTime initialDate = DateTime(2016, DateTime.january, 15);
+        DateTime? selectedDate;
+
+        await tester.pumpWidget(calendarDatePicker(
+          initialDate: initialDate,
+          onDateChanged: (DateTime value) {
+            selectedDate = value;
+          },
+        ));
+
+        final bool isToday = DateUtils.isSameDay(initialDate, selectedDate);
+        final String semanticLabelSuffix = isToday ? ', ${localizations.currentDateLabel}' : '';
+
+        // The initial date should be announced.
+        expect(
+          tester.takeAnnouncements().last.message,
+          '${localizations.formatFullDate(initialDate)}$semanticLabelSuffix',
+        );
+
+        // Select a new date.
+        await tester.tap(find.text('20'));
+        await tester.pumpAndSettle();
+
+        // The selected date should be announced.
+        expect(
+          tester.takeAnnouncements().last.message,
+          '${localizations.selectedDateLabel} ${localizations.formatFullDate(selectedDate!)}$semanticLabelSuffix',
+        );
+
+        // Select the initial date.
+        await tester.tap(find.text('15'));
+
+        // The initial date should be announced as selected.
+        expect(
+          tester.takeAnnouncements().first.message,
+          '${localizations.selectedDateLabel} ${localizations.formatFullDate(initialDate)}$semanticLabelSuffix',
+        );
+
+        semantics.dispose();
+      }, variant: TargetPlatformVariant.desktop());
     });
   });
 

packages/flutter/test/material/localizations_test.dart

@@ -137,6 +137,7 @@ void main() {
     expect(localizations.currentDateLabel, isNotNull);
     expect(localizations.scrimLabel, isNotNull);
     expect(localizations.bottomSheetLabel, isNotNull);
+    expect(localizations.selectedDateLabel, isNotNull);
 
     expect(localizations.scrimOnTapHint('FOO'), contains('FOO'));