display current build on editor download button

src/site/downloads.html

 ---
 layout: default
 title: "Downloads & Source"
 description: "Download Dart Editor, the Dart SDK, and Dartium. Also, get the source code for the Dart project."
 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>
 
 <p>
   Choose your OS:
   {% include os-choices.html %}
 </p>
@@ skipping 82 matching lines @@
       the source using the project build tools and scripts
       </a>
     </li>
     <li>
       <a href="http://code.google.com/p/dart/wiki/Contributing">Contribute</a>
       patches to the Dart project
     </li>
   </ul>
 
 </section>