title: Angular async validator's state not reflected in form field's error state
body: <p>I have an an async validator on a <code>zip</code> field:</p>
  
  <pre><code>zip: ['', {
              validators: [
                Validators.required,
                Validators.minLength(5),
                Validators.maxLength(5)
              ],
              asyncValidators: [
                adPostFormValidators.isValidZip(this.locationService)
              ]
            },
        ],
  </code></pre>
  
  <p>However, the field doesn't seem to reflect the error that comes from the async validator until I click out of the field. For example, this is before I click out (the <code>null</code> is the field's error state):</p>
  
  <p><a href="https://i.stack.imgur.com/ZptTk.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ZptTk.png" alt="enter image description here"></a></p>
  
  <p>I know that the async validator has run because I output its results into the console:</p>
  
  <p><a href="https://i.stack.imgur.com/FmfY4.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/FmfY4.png" alt="enter image description here"></a></p>
  
  <p>Then, when I click out or lose focus, the error state is now accurate:</p>
  
  <p><a href="https://i.stack.imgur.com/ZvQXs.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ZvQXs.png" alt="enter image description here"></a></p>
  
  <p>However, the validator was not run again, as nothing new was logged into the console.</p>