appropriating Shailen's excellent Futures intro into tutorial

src/site/docs/tutorials/polymer-intro/index.markdown

 ---
 layout: default
 title: "Define a Custom Element"
 description: "Create a custom HTML element using Polymer."
 has-permalinks: true
 tutorial:
   id: polymer-intro
-next: fetchdata
-next-title: "Fetch Data Dynamically"
-prev: shared-pkgs
+next: futures/
+next-title: "Use Future-Based APIs"
+prev: shared-pkgs/
 prev-title: "Install Shared Packages"
 ---
 
 {% capture whats_the_point %}
 
 * Polymer.dart is the next evolution of Web UI.
 * Everything in Polymer.dart is an element.
 * Custom elements provide semantically meaningful encapsulation.
 * Use Polymer.dart to build custom elements.
 * Bind Dart data to HTML elements.
@@ skipping 509 matching lines @@
 
 The next tutorial,
 [Fetch Data Dynamically](/docs/tutorials/fetchdata/),
 shows you how to fetch data
 and use JSON to encode and decode
 that data.
 
 {% endcapture %}
 
 {% include tutorial.html %}