title: How do you add an initial selection for Angular Material Table SelectionModel?
body: <p>The Angular Material documentation gives a nice example for how to add selection to a table (<a href="https://material.angular.io/components/table/overview#selection" rel="noreferrer">Table Selection docs</a>).  They even provide a <a href="https://stackblitz.com/angular/aypedlrjldj?file=app%2Ftable-selection-example.ts" rel="noreferrer">Stackblitz</a> to try it out.</p>
  
  <p>I found in the <a href="https://github.com/angular/material2/blob/master/src/cdk/collections/selection.ts" rel="noreferrer">code for the SelectionModel constructor</a> that the first argument is whether there can be multiple selections made (true) or not (false). The second argument is an array of initially selected values.</p>
  
  <p>In the demo, they don't have any initially selected values, so the second argument in their constructor (line 36) is an empty array (<code>[]</code>).</p>
  
  <p>I want to change it so that there is an initially selected value, so I changed line 36 to:</p>
  
  <pre><code>selection = new SelectionModel&lt;PeriodicElement&gt;(true, [{position: 2, name: 'Helium', weight: 4.0026, symbol: 'He'}]);
  </code></pre>
  
  <p>This changes the checkbox in the header to an indeterminate state (as expected), but does not cause the row in the table to be selected.  Am I setting the initial value incorrectly, or what am I missing here?  How can I set an initially selected value?</p>