title: Mat-tree create a tree with values to display and ids
body: <p>I'm learning how to use mat-tree following the docs, but now I need to use a data source tree that has not only strings defined in it, because when I click a node I need to know its id.<br>
  <a href="https://stackblitz.com/edit/angular-e5wsql" rel="nofollow noreferrer">This is the stackblitz of my project</a>.<br>
  Basically I need to use this tree with checkboxes that I've built following the docs, but now my data source is this: </p>
  
  <pre><code>const TREE_DATA:TodoItemNode = {
    item: "First item",
    children: [{
      item: 'Second item',
      children:[
        {
          item: "Third item",
          id: 3
        }
      ],
        id:2
      }],
    id:1
    }
  </code></pre>
  
  <p>So I want to display as text the value of item and then get the "id" when that node is clicked. Currently I can't make it work and I only see these tree nodes:</p>
  
  <p><a href="https://i.stack.imgur.com/uLANB.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/uLANB.png" alt="enter image description here"></a></p>
  
  <p>Is there a working example for data as JSON, with properties etc. as I want to achieve?</p>