title: multiple mat-select start with all checked?
body: <p>I have a dinamicly mat-select in an Angular4 app. What I need is: when I fill the mat-select options, I need to set all the options as checked. I tried many thins but no one worked. Please help.</p>
  
  <p>This is what I got:</p>
  
  <p>ts:</p>
  
  <pre><code>options = {
      0: 'op1',
      1: 'op2',
      2: 'op3',
      3: 'op4'
    };
  </code></pre>
  
  <p>but the <code>options</code> variable can change. Sometimes the length can be 5, sometimes can be 16, sometimes 0, that is dinamic.</p>
  
  <p>Then, in my HTML i got this:</p>
  
  <pre><code>&lt;mat-select multiple placeholder="Options" required="true" [(ngModel)]="selected-options"&gt;
    &lt;mat-option *ngFor="let op of options" [value]="op"&gt;{{ op }}&lt;/mat-option&gt;
  &lt;/mat-select&gt;
  </code></pre>
  
  <p>Look the <code>multiple</code> in the mat-select tag.</p>
  
  <p>THIS WORKS! but that just set the options in the select and the value is unchecked. </p>
  
  <p>So... how can I set all the items checked?</p>