title: mat-select required validation not working
body: <p>I have the following code </p>
  
  <pre><code>&lt;form #createForm="ngForm"&gt;
   &lt;mat-form-field&gt;
     &lt;mat-select placeholder="Favorite food"
        matInput
       [ngModel]
       food="food"
       #food="ngModel" required&gt;
      &lt;mat-option *ngFor="let food of foods" [value]="food.value"&gt;
        {{ food.viewValue }}
      &lt;/mat-option&gt;
     &lt;/mat-select&gt;
  &lt;/mat-form-field&gt;
  &lt;/form&gt;
  &lt;button [disabled]="!createForm.valid"&gt;submit&lt;/button&gt;
  </code></pre>
  
  <p>Since I want the "selection" is a required field, the "submit" button should be disabled when the form is rendered. However, the "submit" button is enabled when the form is displayed. What is the problem?</p>