[polymer] update pubspecs to prepare for -pre.0 release

R=sigmund@google.com

Review URL: https://codereview.chromium.org//180213007

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/web_components@33178 260f80e4-7a28-3924-810f-c04153c831b5
diff --git a/pubspec.yaml b/pubspec.yaml
index 92762ff..7dc6f80 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -8,4 +8,4 @@
   standard tag. Shadow DOM is designed to provide encapsulation for custom
   elements, by hiding DOM subtrees under shadow roots.
 environment:
-  sdk: ">=1.2.0-dev.4.0 <2.0.0"
+  sdk: ">=1.2.0 <2.0.0"