display current build on editor download button

src/site/docs/editor/getting-started/index.html

 ---
 layout: default
 title: "Getting Started with Dart Editor"
 description: "Step-by-step instructions for installing Dart Editor and using it to create and run Dart apps."
 js:
 - url: /js/os-switcher.js
   defer: true
 - url: /js/editor-downloads-analytics.js
   defer: true
+- url: /js/editor-version.js
+  defer: true
 ---
 
 {% include platform-specific-text/_noscript.html %}
 
 <h1> {{ page.title }} </h1>
 
 <section>
 
 <p>
 Follow this tutorial to install and use Dart Editor on
-    <input type="radio" name="osChoices" id="win">
-    <label class="os-choice" for="win">Windows</label>,
-
-    <input type="radio" name="osChoices" id="linux" checked="true">
-    <label class="os-choice" for="linux">Linux</label>, or
-
-    <input type="radio" name="osChoices" id="mac">
-    <label class="os-choice" for="mac">Mac</label>.
+{% include os-choices.html %}
 </p>
 
 <p>
 Dart Editor is a lightweight, open-source editor for Dart code.
 You can use it to edit and run Dart apps,
 as well as to invoke the Dart-to-JavaScript compiler.
 For more information about what Dart Editor can do,
 see the <a href="/docs/editor/">Dart Editor homepage</a>.
 If you run into a problem, see
 <a href="/docs/editor/troubleshoot.html">Troubleshooting Dart Editor</a>.
@@ skipping 286 matching lines @@
 We'll open a new issue for you, if appropriate,
 without disclosing your sensitive or personally identifiable information.
 </p>
 
 <p>
 Or you can just <a href="http://code.google.com/p/dart/issues/entry?template=Defect%20report%20from%20user&amp;labels=Area-Editor,Type-Defect,Priority-Medium">create
 a new Dart Editor issue.</a>
 </p>
 
 </section>