title: Connected callback is not being called while creating custom html elements
body: <p>I was trying to create a custom tag in HTML using JavaScript. I want to create the custom element using ES6 JavaScript syntax. I have written this code to create the custom element:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>customElements.define('neo-element', NeoElement);
  function NeoElement (){
      var ref =  Reflect.construct(HTMLElement,[], this.constructor) ;
      return ref;
  };
  NeoElement.prototype = Object.create(HTMLElement.prototype);
  NeoElement.prototype.constructor = NeoElement;
  NeoElement.prototype.connectedCallback = function(){
      this.innerHTML = `&lt;h1&gt;Hello world&lt;/h1&gt;`;
  };</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;neo-element&gt;&lt;/neo-element&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>I have verified that NeoElement is extending HTMLElement properly, but still nothing is getting printed inside the <code>&lt;neo-element&gt;</code> tag.</p>
  
  <p>Could anyone look at the code and tell me what am I missing in ES5 syntax ?</p>