first draft of polymer element tutorial

src/site/docs/tutorials/shared-pkgs/index.markdown

 ---
 layout: default
 title: "Install Shared Packages"
 description: "Packages are bundles of source code, tools, and resources that help you to organize and share code"
 has-permalinks: true
 tutorial:
   id: packages
-next: web-ui/
-next-title: "Get Started with Web UI"
+next: polymer-intro/
+next-title: "Define a Custom Element"
 prev: remove-elements/
 prev-title: "Remove DOM Elements"
 ---
 
 {% capture whats_the_point %}
 
 * Packages are awesome.
 * Packages are cool.
 * Share your code in packages, with all your friends at school.
 
@@ skipping 279 matching lines @@
     <a href="https://pub.dartlang.org/packages/web_ui">web_ui</a>&mdash;a package
     created by the Dart team that lets you use Web components and templating.
     The
     <a href="/docs/tutorials/web-ui/">next target</a> covers Web UI.
   </li>
 </ul>
 
 {% endcapture %}
 
 {% include tutorial.html %}