title: Office Dialog API send message to child dialog
body: <p>We would like to use the Dialog API for some extra operations which are better suited to a larger work area. We already have the ability via the API to message the parent from the dialog (<code>Office.context.ui.messageParent</code>), however we would also like the ability to directly message the dialog, like a <code>sendMessage</code> function, for example:</p>
  
  <pre><code>var dialog;
  Office.context.ui.displayDialogAsync('https://myDomain/myDialog.html', function (asyncResult) {
      if (asyncResult.status === "failed") { 
          showNotification(asynceResult.error.code = ": " + asyncResult.error.message); 
      } else {
          dialog = asyncResult.value;
          dialog.addEventHandler(Office.EventType.DialogMessageReceived, processMessage);
          dialog.sendMessage({ /* my payload */ }); //???
      }
  }); 
  </code></pre>
  
  <p>There are a couple of options to pass data from the parent to the child:</p>
  
  <ol>
  <li>query string: <code>displayDialogAsync(myUrl + "?myMessage=...", function...)</code></li>
  <li>localStorage (if on the same domain): <code>localStorage.setItem("DIALOG_MESSAGE", myMessage)</code></li>
  </ol>
  
  <p>However, both of these can only really be used on initialization of the dialog, they do not provide a nice way to do asynchronous messaging.</p>