Upgrade video-player package version to pick up some strong mode error
fixes.

examples/flutter_gallery/pubspec.yaml

diff --git a/examples/flutter_gallery/pubspec.yaml b/examples/flutter_gallery/pubspec.yaml
index d127050..e3b8174 100644
--- a/examples/flutter_gallery/pubspec.yaml
+++ b/examples/flutter_gallery/pubspec.yaml
@@ -9,7 +9,7 @@
   string_scanner: 1.0.2
   url_launcher: 2.0.1
   cupertino_icons: 0.1.1
-  video_player: 0.2.0
+  video_player: 0.2.1
 
   # Also update dev/benchmarks/complex_layout/pubspec.yaml
   flutter_gallery_assets: