introduce web_components pkg for consolidated polyfills

pkg/polymer/pubspec.yaml

Index: pkg/polymer/pubspec.yaml
diff --git a/pkg/polymer/pubspec.yaml b/pkg/polymer/pubspec.yaml
index 15475869ff7d21fc3e07f183722edb256e1758e7..08e2b6ad5024aec0f1949d0ae051967e0f5c19bd 100644
--- a/pkg/polymer/pubspec.yaml
+++ b/pkg/polymer/pubspec.yaml
@@ -11,16 +11,14 @@ dependencies:
   args: ">=0.9.0 <0.10.0"
   barback: ">=0.9.0 <0.12.0"
   browser: ">=0.9.0 <0.10.0"
-  custom_element: ">=0.9.1+1 <0.10.0"
-  html5lib: ">=0.9.1 <0.10.0"
-  html_import: ">=0.9.0 <0.10.0"
+  html5lib: ">=0.9.2-dev <0.10.0"
   logging: ">=0.9.0 <0.10.0"
-  observe: ">=0.9.1 <0.10.0"
+  observe: ">=0.10.0-dev <0.11.0"
   path: ">=0.9.0 <2.0.0"
-  polymer_expressions: ">=0.9.1 <0.10.0"
-  shadow_dom: ">=0.9.1 <0.10.0"
+  polymer_expressions: ">=0.10.0-dev <0.11.0"
   source_maps: ">=0.9.0 <0.10.0"
-  template_binding: ">=0.9.0 <0.10.0"
+  template_binding: ">=0.10.0-dev <0.11.0"
+  web_components: ">=0.3.0 <0.4.0"
   yaml: ">=0.9.0 <0.10.0"
 transformers:
 - observe: