title: How to use ngClass to mark a field as valid/invalid in ng6
body: <p>how can I apply "is-invalid" via ngClass to a bound field that angular 6 is currently marking as ng-invalid.</p>
  
  <p>Would like to use Bootstraps is-valid/is-invalid css classes to improve view of the form.</p>
  
  <p>Thanks</p>
  
  <p>Input is as below</p>
  
  <pre><code>&lt;input type="text"
    id="text-input"
    name="text-input"
    class="form-control"
    placeholder="Provide a Name"
    formControlName="elementName"
    [(ngModel)]="stepName"&gt;
  </code></pre>