Preparing versions to release polymer 0.12.0

R=jakemac@google.com

Review URL: https://codereview.chromium.org//423233004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/web_components@38734 260f80e4-7a28-3924-810f-c04153c831b5
diff --git a/pubspec.yaml b/pubspec.yaml
index 8c21258..cf4a1ee 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: web_components
-version: 0.5.0-dev
+version: 0.5.0
 author: Polymer.dart Authors <web-ui-dev@dartlang.org>
 homepage: https://www.dartlang.org/polymer-dart/
 description: >