Add CloseButtonTooltip to the 'X' button on a SnackBar (#143934)

Adds a localized Close Button tooltip to the 'X' Button on a SnackBar, making it readable by screen readers.

Github Issue #143793 

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

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

@@ -11,6 +11,7 @@ import 'colors.dart';
 import 'icon_button.dart';
 import 'icons.dart';
 import 'material.dart';
+import 'material_localizations.dart';
 import 'material_state.dart';
 import 'scaffold.dart';
 import 'snack_bar_theme.dart';
@@ -656,6 +657,7 @@ class _SnackBarState extends State<SnackBar> {
             iconSize: 24.0,
             color: widget.closeIconColor ?? snackBarTheme.closeIconColor ?? defaults.closeIconColor,
             onPressed: () => ScaffoldMessenger.of(context).hideCurrentSnackBar(reason: SnackBarClosedReason.dismiss),
+            tooltip: MaterialLocalizations.of(context).closeButtonTooltip,
           )
         : null;
 

packages/flutter/test/material/snack_bar_test.dart

@@ -3243,6 +3243,40 @@ void main() {
             'snack_bar.goldenTest.floatingWithActionWithIcon.png'));
   });
 
+  testWidgets('SnackBar has tooltip for Close Button', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/143793
+    await tester.pumpWidget(MaterialApp(
+      theme: ThemeData(useMaterial3: false),
+      debugShowCheckedModeBanner: false, // https://github.com/flutter/flutter/issues/143616
+      home: const Scaffold(
+        bottomSheet: SizedBox(
+          width: 200,
+          height: 50,
+          child: ColoredBox(
+            color: Colors.pink,
+          ),
+        ),
+      ),
+    ));
+
+    final ScaffoldMessengerState scaffoldMessengerState = tester.state(find.byType(ScaffoldMessenger));
+    scaffoldMessengerState.showSnackBar(
+      SnackBar(
+        content: const Text('Snackbar with close button'),
+        duration: const Duration(days: 365),
+        action: SnackBarAction(label: 'ACTION', onPressed: () {}),
+        behavior: SnackBarBehavior.floating,
+        showCloseIcon: true,
+      ),
+    );
+    await tester.pumpAndSettle(); // Have the SnackBar fully animate in.
+
+    expect(
+      find.byTooltip(MaterialLocalizations.of(scaffoldMessengerState.context).closeButtonLabel),
+      findsOneWidget
+    );
+  });
+
   testWidgets('Material2 - Fixed width snackbar can display optional icon', (WidgetTester tester) async {
     await tester.pumpWidget(MaterialApp(
       theme: ThemeData(useMaterial3: false),