title: How to update mat-tree to handle add children nodes to leaf nodes (convert leaf nodes to parents)
body: <p>I'm using <a href="https://material.angular.io/components/tree/overview" rel="nofollow noreferrer">Angular Material6 Tree component</a> with check boxes.</p>
  
  <p>In the <a href="https://stackblitz.com/angular/aaaxbedprnre?file=app%2Ftree-checklist-example.ts" rel="nofollow noreferrer">Original Example</a>, We only can add new child to parent Node. 
  What If I want to add a new child to a <a href="https://stackblitz.com/edit/angular-ax72ew?file=app%2Ftree-checklist-example.html" rel="nofollow noreferrer">leaf node</a>. </p>