title: ES6 String Substitution does not work
body: <p>test.html:</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
  &lt;head&gt;
      &lt;title&gt;Test&lt;/title&gt;
  &lt;/head&gt;
  &lt;body&gt;
      &lt;script src="test.js"&gt;&lt;/script&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>test.js:</p>
  
  <pre><code>let fName = "Nick";
  let lName = "Lee";
  
  document.write('${fName}');
  </code></pre>
  
  <p>Output on Chrome:</p>
  
  <p>${fName}</p>
  
  <p>I expected it to be Nick. </p>