Release 0.12.0-beta.6.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1079763002
diff --git a/pubspec.yaml b/pubspec.yaml
index 8f012b0..f28b7cb 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: test
-version: 0.12.0-dev
+version: 0.12.0-beta.6
 author: Dart Team <misc@dartlang.org>
 description: A library for writing dart unit tests.
 homepage: https://github.com/dart-lang/test