[Material] modernize Typography._withPlatform with Dart 3 switch expression (#181398)

Continuing the effort to clean up some of the older switch statements,
I've moved over to `typography.dart`

I found that `Typography._withPlatform` was using a standard switch
block to handle platform-specific theme assignments. Since both `black`
and `white` themes are resolved based on the same platform variable,
I've refactored this into a single switch expression using a Record.
This allowed me to group Android and Fuchsia together using the new
logical OR pattern, which feels much cleaner.

The logic and the existing fallbacks haven't changed at all—the `assert`
at the start already guarantees we have the required themes if the
platform is null, so I've handled that case directly in the expression.

I've run the local tests in `typography_test.dart` and everything is
passing.

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we expect every widget to implement].
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- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.

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

@@ -215,27 +215,16 @@ class Typography with Diagnosticable {
     TextTheme tall,
   ) {
     assert(platform != null || (black != null && white != null));
-    switch (platform) {
-      case TargetPlatform.iOS:
-        black ??= blackCupertino;
-        white ??= whiteCupertino;
-      case TargetPlatform.android:
-      case TargetPlatform.fuchsia:
-        black ??= blackMountainView;
-        white ??= whiteMountainView;
-      case TargetPlatform.windows:
-        black ??= blackRedmond;
-        white ??= whiteRedmond;
-      case TargetPlatform.macOS:
-        black ??= blackRedwoodCity;
-        white ??= whiteRedwoodCity;
-      case TargetPlatform.linux:
-        black ??= blackHelsinki;
-        white ??= whiteHelsinki;
-      case null:
-        break;
-    }
-    return Typography._(black!, white!, englishLike, dense, tall);
+    final (TextTheme blackResolved, TextTheme whiteResolved) = switch (platform) {
+      TargetPlatform.iOS => (black ?? blackCupertino, white ?? whiteCupertino),
+      TargetPlatform.android ||
+      TargetPlatform.fuchsia => (black ?? blackMountainView, white ?? whiteMountainView),
+      TargetPlatform.windows => (black ?? blackRedmond, white ?? whiteRedmond),
+      TargetPlatform.macOS => (black ?? blackRedwoodCity, white ?? whiteRedwoodCity),
+      TargetPlatform.linux => (black ?? blackHelsinki, white ?? whiteHelsinki),
+      null => (black!, white!),
+    };
+    return Typography._(blackResolved, whiteResolved, englishLike, dense, tall);
   }
 
   const Typography._(this.black, this.white, this.englishLike, this.dense, this.tall);