Prepare 2.9.0 release (#491)

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index 25aebf1..17eb626 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,7 +1,7 @@
 name: gallery
 description: A resource to help developers evaluate and use Flutter.
 repository: https://github.com/flutter/gallery
-version: 2.8.1+020801
+version: 2.9.0+020900
 
 environment:
   sdk: ">=2.6.0 <3.0.0"