title: Angular Reactive Form Disabled Control is not valid
body: <p>I thought that a <code>disabled</code> control would not get validated. But I'm facing this issue.</p>
  <p>I have reproduced using the official Angular reactive form sample:</p>
  <p><a href="https://stackblitz.com/edit/angular-hyvj24?file=src/app/dynamic-form-question.component.ts" rel="noreferrer">https://stackblitz.com/edit/angular-hyvj24?file=src/app/dynamic-form-question.component.ts</a></p>
  <p>If you type something in the email field and focus out, this will trigger the <code>change</code> event in which I'm doing :</p>
  <pre class="lang-js prettyprint-override"><code>this.form.controls['firstName'].disable();
  </code></pre>
  <p><a href="https://i.stack.imgur.com/HfBFm.png" rel="noreferrer"><img src="https://i.stack.imgur.com/HfBFm.png" alt="enter image description here" /></a></p>
  <p>And although, firstname is NOT a required field, it gets validated and we see a red message.
  Without this line of code, firstName can be blank and we'll still be able to save.</p>
  <p>Why is that ?</p>