Update for Flutter 3.10 beta (#1746)

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] All existing and new tests are passing.

---------

Co-authored-by: David Iglesias <ditman@gmail.com>
Co-authored-by: Mark Thompson <2554588+MarkTechson@users.noreply.github.com>
Co-authored-by: John Ryan <ryjohn@google.com>

platform_design/lib/widgets.dart

@@ -19,15 +19,15 @@ class PlatformWidget extends StatelessWidget {
 
   @override
   Widget build(context) {
-    switch (defaultTargetPlatform) {
-      case TargetPlatform.android:
-        return androidBuilder(context);
-      case TargetPlatform.iOS:
-        return iosBuilder(context);
-      default:
-        assert(false, 'Unexpected platform $defaultTargetPlatform');
-        return const SizedBox.shrink();
-    }
+    assert(
+        defaultTargetPlatform == TargetPlatform.android ||
+            defaultTargetPlatform == TargetPlatform.iOS,
+        'Unexpected platform $defaultTargetPlatform');
+    return switch (defaultTargetPlatform) {
+      TargetPlatform.android => androidBuilder(context),
+      TargetPlatform.iOS => iosBuilder(context),
+      _ => const SizedBox.shrink()
+    };
   }
 }