title: property selectedIndex does not exists on type HTMLElement
body: <p>I'm trying to convert some js files to using typescript in asp.net webforms.  The javascript works but it won't compile in ts because of the errors.  Here is the code</p>
  
  <pre><code> var mDropDownID = arySecondTier[x];
   var DropdownList = (document.getElementById("ctl00_ContentPlaceHolder1_lvCASec2_Detail_ctrl" + mDropDownID + "_ddlPoints"));
   var SelectedIndex = DropdownList.selectedIndex;
  </code></pre>
  
  <p>the error happens on the selectedIndex line.  From what i've read with typescript there is no selectedIndex on the HTMLElement, that i need to use a subtype of HTMLElement.  If that is the case i'm having problems finding what the name of that subtype would be.  If it's something else, well then i'd love to know what.  </p>
  
  <p>I've seen some who convert the DropDownList to , but my first thought is doesn't that go against the reason you want to use typescript.  </p>
  
  <p>Thanks</p>