Step one towards stable error messages with details:

pkg/polymer/pubspec.yaml

Index: pkg/polymer/pubspec.yaml
diff --git a/pkg/polymer/pubspec.yaml b/pkg/polymer/pubspec.yaml
index bb9e609afcaad9b132b8282844b813f8d4179457..ac7b8224679f415479a8ca97bac6b3acdb81e199 100644
--- a/pkg/polymer/pubspec.yaml
+++ b/pkg/polymer/pubspec.yaml
@@ -1,5 +1,5 @@
 name: polymer
-version: 0.13.0+2
+version: 0.13.1-dev
 author: Polymer.dart Authors <web-ui-dev@dartlang.org>
 description: >
   Polymer.dart is a new type of library for the web, built on top of Web
@@ -11,7 +11,7 @@ dependencies:
   args: '>=0.10.0 <0.13.0'
   barback: '>=0.14.2 <0.16.0'
   browser: '>=0.10.0 <0.11.0'
-  code_transformers: '>=0.2.0 <0.3.0'
+  code_transformers: '>=0.2.2-dev <0.3.0'
   html5lib: '>=0.12.0 <0.13.0'
   logging: '>=0.9.2 <0.10.0'
   observe: '>=0.11.0 <0.13.0'
@@ -25,6 +25,7 @@ dependencies:
   yaml: '>=0.9.0 <3.0.0'
 dev_dependencies:
   unittest: '>=0.10.0 <0.12.0'
+  markdown: '>=0.7.0 <0.8.0'
 transformers:
 - polymer/src/build/mirrors_remover:
     $include: lib/polymer.dart