Bring Cloud Run up to Dart 2.8.1 (#528)

diff --git a/cloud_run.Dockerfile b/cloud_run.Dockerfile
index d5b9f7f..7c15cf0 100644
--- a/cloud_run.Dockerfile
+++ b/cloud_run.Dockerfile
@@ -1,5 +1,5 @@
 # Keep aligned with min SDK in pubspec.yaml and Dart test version in .travis.yml
-FROM google/dart:2.7.2
+FROM google/dart:2.8.1
 
 # The specific commit that dart-services should use. This should be kept
 # in sync with the flutter submodule in the dart-services repo.