add explicit const keywords (#23652)

packages/flutter_tools/test/devfs_test.dart

diff --git a/packages/flutter_tools/test/devfs_test.dart b/packages/flutter_tools/test/devfs_test.dart
index 8aebeb0..e1502ec 100644
--- a/packages/flutter_tools/test/devfs_test.dart
+++ b/packages/flutter_tools/test/devfs_test.dart
@@ -71,7 +71,7 @@
       file.parent.createSync(recursive: true);
       file.writeAsBytesSync(<int>[1, 2, 3]);
 
-      final DateTime fiveSecondsAgo = DateTime.now().subtract(Duration(seconds:5));
+      final DateTime fiveSecondsAgo = DateTime.now().subtract(const Duration(seconds:5));
       expect(content.isModifiedAfter(fiveSecondsAgo), isTrue);
       expect(content.isModifiedAfter(fiveSecondsAgo), isTrue);
       expect(content.isModifiedAfter(null), isTrue);