Update polymer tutorial; make directory paths match new sample structure

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

 ---
 layout: tutorial
 title: "Use Future-Based APIs"
 description: "A first look at Futures and how to use them to make your asynchronous code better."
 has-permalinks: true
 tutorial:
   id: futures
 next: streams/
 next-title: "Use Streams for Data"
 prev: polymer-intro/
@@ skipping 10 matching lines @@
 * Call `then()` to schedule code that runs when a Future completes.
 * Use `catchError()` to handle errors that occur within a Future.
 * You can chain Futures to run asynchronous functions in order.
 
 {% endcapture %}
 
 {% capture sample_links %}
 
 
 <p markdown="1"> This tutorial features these examples
-in the `bin` directory,
+in the `futures/bin` directory,
 all variations of the same example:</p>
 
-* futures1
-* futures2
-* futures3
-* futures4
-* futures5
+* futures1.dart
+* futures2.dart
+* futures3.dart
+* futures4.dart
+* futures5.dart
 
 <p>
 Don't have the source code?
 <a href="https://github.com/dart-lang/dart-tutorials-samples/archive/master.zip">
   Download it.
 </a>
 
 {% endcapture %}
 
 {% capture content %}
@@ skipping 223 matching lines @@
 uses a Future when doing an HTTP request.
 
 * The example featured in
 [Use IndexedDB](/docs/tutorials/indexeddb/)
 uses many Futures when interacting with the database.
 
 {% endcapture %}
 
 {% include tutorial.html %}