appropriating Shailen's excellent Futures intro into tutorial

src/site/docs/tutorials/fetchdata/index.markdown

 ---
 layout: default
 title: "Fetch Data Dynamically"
 description: "Use HttpRequest to fetch data from a file or a server."
 has-permalinks: true
 tutorial:
   id: fetchdata
 next: forms/
 next-title: "Get Input from a Form"
-prev: polymer-intro/
-prev-title: "Define a Custom Element"
+prev: futures/
+prev-title: "Use Future-Based APIs"
 ---
 
 {% capture whats_the_point %}
 
 * Data on the web is often formatted in JSON.
 * JSON is text based and human readable.
 * The dart:convert library provides support for JSON.
 * Use HttpRequest to dynamically load data.
 
 {% endcapture %}
@@ skipping 268 matching lines @@
 
 This program uses a convenience method,
 `getString()`,
 provided by the HttpRequest class
 to request the file from the server.
 
 <img class="scale-img-max" src="images/getstringfunction.png"
      alt="Use the getString() function to make a request">
 
 The getString() method uses a Future object to handle the request.
-You can use the code above as an idiom
+A Future is a way to perform potentially time-consuming operations,
+such as HTTP requests, asynchronously.
+If you haven't encountered Futures yet,
+you can learn more about them in
+[Use Future-Based APIs](/docs/tutorials/futures/).
+Until then, you can use the code above as an idiom
 and provide your own code for the body of the processString() function
 and your own code to handle the error.
 
 ###Using a relative URI
 
 The URI used for the GET request specifies just the name of
 the portmanteaux_simple.json data file.
 Let's take a look at how that works.
 
 
@@ skipping 144 matching lines @@
      alt="Decode a JSON formatted list of strings">
 
 ##Other resources
 
 Check out Chris Buckett's article,
 <a href="/articles/json-web-service/"
    target="_blank">Using Dart with JSON Web Services</a>,
 for more information and an example with source code for both
 client and server programs.
 
-##What Next?
+##What next?
 
-The next tutorial,
+* If you skipped the previous tutorial,
+[Use Future-Based APIs](/docs/tutorials/futures/),
+we highly recommend that you go back and learn about Futures
+before going any further.
+
+* The next tutorial,
 [Get Input from a Form](/docs/tutorials/forms/),
 contains a client/server example that
 shows you how to use a form to get data from the user,
 and using JSON, send that form to a server,
 and handle the server's response.
 
 {% endcapture %}
 
 {% include tutorial.html %}