enable lint prefer_generic_function_type_aliases (#21680)

dev/manual_tests/lib/material_arc.dart

diff --git a/dev/manual_tests/lib/material_arc.dart b/dev/manual_tests/lib/material_arc.dart
index e4afb81..06cdf9f 100644
--- a/dev/manual_tests/lib/material_arc.dart
+++ b/dev/manual_tests/lib/material_arc.dart
@@ -396,7 +396,7 @@
   }
 }
 
-typedef Widget _DemoBuilder(_ArcDemo demo);
+typedef _DemoBuilder = Widget Function(_ArcDemo demo);
 
 class _ArcDemo {
   _ArcDemo(this.title, this.builder, TickerProvider vsync)