introduce web_components pkg for consolidated polyfills

pkg/polymer/pubspec.yaml

 name: polymer
 version: 0.10.0-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
   Components, and designed to leverage the evolving web platform on modern
   browsers.
 homepage: https://www.dartlang.org/polymer-dart/
 dependencies:
   analyzer: ">=0.10.1 <0.11.0"
   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:
     files: lib/src/instance.dart
 dev_dependencies:
   unittest: ">=0.9.0 <0.10.0"
 environment:
   sdk: ">=1.1.0-dev.5.0 <2.0.0"