title: Issue communication with postMessage from parent to child iFrame
body: <p>I'm having an issue communicating from my parent window to the child iFrame. But in the other side, everything works perfectly. 
  Here is how I get the chil iFrame object in order to fire the <strong><em>postMessage</em></strong> function:</p>
  
  <pre><code>var iFrame = document.getElementById('Frame').contentWindow;
  </code></pre>
  
  <p>When I print it int he console, I get the following:</p>
  
  <pre><code>Window {parent: Window, opener: null, top: Window, length: 0, frames: Window…}
  </code></pre>
  
  <p>When I proceed to the postMessage function as follows:</p>
  
  <pre><code>iFrame.postMessage("message", "http://contoso.com");
  </code></pre>
  
  <p>It shows me an error when loading the page: <strong><em>iFrame.postMessage is not a function</em></strong>.
  When I execute the postMessage in console, I get an <strong><em>undefined</em></strong></p>
  
  <p>What am I doing wrong ? </p>