Declare locals final where not reassigned (gallery) (#8571)

diff --git a/examples/flutter_gallery/lib/demo/material/cards_demo.dart b/examples/flutter_gallery/lib/demo/material/cards_demo.dart
index cba2d80..72a23a0 100644
--- a/examples/flutter_gallery/lib/demo/material/cards_demo.dart
+++ b/examples/flutter_gallery/lib/demo/material/cards_demo.dart
@@ -45,9 +45,9 @@
 
   @override
   Widget build(BuildContext context) {
-    ThemeData theme = Theme.of(context);
-    TextStyle titleStyle = theme.textTheme.headline.copyWith(color: Colors.white);
-    TextStyle descriptionStyle = theme.textTheme.subhead;
+    final ThemeData theme = Theme.of(context);
+    final TextStyle titleStyle = theme.textTheme.headline.copyWith(color: Colors.white);
+    final TextStyle descriptionStyle = theme.textTheme.subhead;
 
     return new SizedBox(
       height: height,