Intercept SystemSound.play platform message before it's sent. (#13342)

Fixes https://github.com/flutter/flutter/issues/43462

lib/web_ui/lib/src/engine/window.dart

@@ -141,6 +141,9 @@ class EngineWindow extends ui.Window {
             domRenderer.setTitle(arguments['label']);
             domRenderer.setThemeColor(ui.Color(arguments['primaryColor']));
             return;
+          case 'SystemSound.play':
+            // There are no default system sounds on web.
+            return;
         }
         break;