Bump Dart SDK to `1.24.0-dev.3.0`. (#10110)

* Bump Dart SDK to `1.24.0-dev.3.0`.

* Fix (mega) gallery creation.

dev/tools/mega_gallery.dart

diff --git a/dev/tools/mega_gallery.dart b/dev/tools/mega_gallery.dart
index fd68187..371d1d6 100644
--- a/dev/tools/mega_gallery.dart
+++ b/dev/tools/mega_gallery.dart
@@ -108,7 +108,7 @@
   // Make sure the imports are not marked as unused.
   ${importRefs.toString().trim()}
 
-  runApp(new GalleryApp());
+  runApp(const GalleryApp());
 }
 ''';