Add support for system alert sound (#62086)

* Add SystemSoundType.alert (supported in the engine in flutter/engine#19970)
* Play system alert sound when user tries to dismiss a non-dismissable modal
  barrier

packages/flutter/lib/src/services/system_sound.dart

@@ -9,9 +9,19 @@ import 'dart:async';
 import 'system_channels.dart';
 
 /// A sound provided by the system.
+///
+/// These sounds may be played with [SystemSound.play].
 enum SystemSoundType {
   /// A short indication that a button was pressed.
   click,
+
+  /// A short system alert sound indicating the need for user attention.
+  ///
+  /// Desktop platforms are the only platforms that support a system alert
+  /// sound, so on mobile platforms (Android, iOS), this will be ignored. The
+  /// web platform does not support playing any sounds, so this will be
+  /// ignored on the web as well.
+  alert,
 }
 
 /// Provides access to the library of short system specific sounds for common
@@ -24,6 +34,9 @@ class SystemSound {
 
   /// Play the specified system sound. If that sound is not present on the
   /// system, the call is ignored.
+  ///
+  /// The web platform currently does not support playing sounds, so this call
+  /// will yield no behavior on that platform.
   static Future<void> play(SystemSoundType type) async {
     await SystemChannels.platform.invokeMethod<void>(
       'SystemSound.play',

packages/flutter/lib/src/widgets/modal_barrier.dart

@@ -7,6 +7,7 @@
 import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/rendering.dart';
+import 'package:flutter/services.dart';
 
 import 'basic.dart';
 import 'container.dart';
@@ -103,6 +104,8 @@ class ModalBarrier extends StatelessWidget {
           onDismiss: () {
             if (dismissible)
               Navigator.maybePop(context);
+            else
+              SystemSound.play(SystemSoundType.alert);
           },
           child: Semantics(
             label: semanticsDismissible ? semanticsLabel : null,

packages/flutter/test/widgets/modal_barrier_test.dart

@@ -8,6 +8,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter/gestures.dart' show kSecondaryButton, PointerDeviceKind;
 
@@ -165,6 +166,32 @@ void main() {
     hovered = false;
   });
 
+  testWidgets('ModalBarrier plays system alert sound when user tries to dismiss it', (WidgetTester tester) async {
+    final List<String> playedSystemSounds = <String>[];
+    try {
+      SystemChannels.platform.setMockMethodCallHandler((MethodCall methodCall) async {
+        if (methodCall.method == 'SystemSound.play')
+          playedSystemSounds.add(methodCall.arguments as String);
+      });
+
+      final Widget subject = Stack(
+        textDirection: TextDirection.ltr,
+        children: <Widget>[
+          tapTarget,
+          const ModalBarrier(dismissible: false),
+        ],
+      );
+
+      await tester.pumpWidget(subject);
+      await tester.tap(find.text('target'));
+      await tester.pumpWidget(subject);
+    } finally {
+      SystemChannels.platform.setMockMethodCallHandler(null);
+    }
+    expect(playedSystemSounds, hasLength(1));
+    expect(playedSystemSounds[0], SystemSoundType.alert.toString());
+  });
+
   testWidgets('ModalBarrier pops the Navigator when dismissed by primary tap', (WidgetTester tester) async {
     final Map<String, WidgetBuilder> routes = <String, WidgetBuilder>{
       '/': (BuildContext context) => const FirstWidget(),