title: Angular mat tree on matTreeNodeDef iteration not able to get index as of ngfor
body: <p>I have an angular material tree. On which i have a code like below</p>
  <pre><code>&lt;mat-tree-node *matTreeNodeDef=&quot;let node; let idx=index&quot;&gt;
    {{node.filename}}::{{idx}}
  &lt;/mat-tree-node&gt;
  </code></pre>
  <p>Here idx not populating</p>