title: iron-ajax hitting an URL gives No 'Access-Control-Allow-Origin' header is present on the requested resource
body: <p>I am using <code>&lt;iron-ajax&gt;</code> like as : </p>
  
  <pre><code>&lt;iron-ajax id="dataAjax" url={{url}} method="POST" handle-as="json" params="{{getParam()}}" on-response="createResp" last-response="{{ajaxResponse2}}"&gt;&lt;/iron-ajax&gt;
  </code></pre>
  
  <p>When i Check in Network tab in Chrome, response is coming, but because of following error <code>createResp</code> does not execute.</p>
  
  <pre><code>XMLHttpRequest cannot load http://...URL.... No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:3000' is therefore not allowed access.
  </code></pre>
  
  <p>I tried with this: </p>
  
  <pre><code>_handleError:function(event){
          console.log(event.detail.request.xhr.response); //null
        }
  </code></pre>
  
  <p>but i got <strong>null</strong>, but in network tab i am able to see the response.</p>
  
  <p>And also status is 200</p>
  
  <p>Can anyone help me how to get the response.</p>