Bump dart 2.9-dev version (#518)

* Flutter version bump, part 2

* bump dart 2.9 dev version
diff --git a/.travis.yml b/.travis.yml
index e84bb64..f89b237 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,7 +5,7 @@
   # Keep this value aligned with min SDK value in pubspec.yaml and FROM line of Dockerfile
   - "2.7.2"
   - "2.8.0-20.11.beta"
-  - "2.9.0-3.0.dev"
+  - "2.9.0-4.0.dev"
 
 # Speed up builds by using containerization. Disable this if you need to use
 # sudo in your scripts.