title: Angular 6 Material Nested Tree is not working with dynamic data
body: <p>I am using <code>mat-tree</code> with <code>mat-nested-tree-node</code> in Angular 6.
  What I want is to load the data dynamically when the user toggles expand icon. </p>
  
  <p>Using the dynamic data example of <code>Flat Tree</code> given in <a href="https://material.angular.io/components/tree/examples" rel="noreferrer">Material Examples</a> I have tried to use the same concept for <code>Nested Tree</code>. This is what I have tried so far <a href="https://stackblitz.com/edit/angular-naarcp" rel="noreferrer">https://stackblitz.com/edit/angular-naarcp</a></p>
  
  <p>But it only shows the data which was prepopulated in the data array although in the console it is clear that data is getting updated but it never gets shown on the UI.</p>
  
  <p>It recursively calls the <code>_getChildren</code> method for the nodes <code>parent, child1, child2, child3</code> because this is initial data. I am adding <code>My Child</code> in <code>child1</code> and <code>child3</code> when user expands it but the added node is never shown.</p>
  
  <p>I can not add dynamic children in <code>_getChildren</code> because it gets called recursively till last node.</p>
  
  <h2>Note:</h2>
  
  <p>I dont want to use Flat tree because it manages everything in single array and updating the single array gets really difficult in asynchronous loading of data</p>
  
  <h2>Help </h2>
  
  <p>Is there anything I am missing or nested trees are designed to work this way?</p>