title: How do I programmatically select Material Tree nodes with SelectionModel?
body: <p>I have a tree with checkboxes <a href="https://stackblitz.com/angular/mndroyqrvbk?file=app%2Ftree-checklist-example.ts" rel="noreferrer">(let's use Material's example here)</a>. I'd like the fruit nodes to begin checked. How do I check those nodes?</p>
  
  <p>I see that <code>SelectionModel</code> has a <code>select()</code> method that the example passes a node. But in the example, the data the tree is made of is an array of <code>TodoItemNode</code>, but the <code>SelectionModel</code> contains <code>TodoItemFlatNode</code>. The <code>transformer</code> method in the example can "flatten" my node (ie. convert <code>TodoItemNode</code> to <code>TodoItemFlatNode</code>), but that would return a new instance.</p>
  
  <p>How can I programmatically select mat-tree checkboxes to match my data?</p>