title: typescript for / of generating invalid javascript
body: <p>I am using typescript in visual studio 2015 in an ASP.NET web application.<br>
  (default typescript settings, version 1.8.36.0 - I always use <code>"use strict"</code> in my <code>.ts</code> files.)  </p>
  
  <p>Given the following code:</p>
  
  <pre><code>for (let file of element.files) {
      file.sourceDocumentId = id;
  }
  </code></pre>
  
  <p>The <code>.js</code> file generated by typescript is as follows.</p>
  
  <pre><code>for (var _i = 0, _a = element.files; _i &lt; _a.length; _i++) {
      var file = _a[_i];
      file.sourceDocumentId = id;
  }
  </code></pre>
  
  <p><strong>Firefox throws an exception indicating that <code>_a</code> is undefined.</strong><br>
  this error seems reasonable - is this a bug in typescript?</p>