Bump version to 0.12.31 (#776)

diff --git a/pubspec.yaml b/pubspec.yaml
index cb7c161..d426652 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: test
-version: 0.12.31-dev
+version: 0.12.31
 author: Dart Team <misc@dartlang.org>
 description: A library for writing dart unit tests.
 homepage: https://github.com/dart-lang/test