title: angular 6 removeAt and trackBy update formArray but not correctly dom
body: <p>I add and delete, <code>formArray</code> in a <code>formGroup</code>.
  I track the id's to delete with <code>trackBy</code>.</p>
  
  <p>The good index pass to the function <code>delete(i)</code> and remove the good <code>formArray</code> element in the form, but in dom, the last one disappears each time.</p>
  
  <p>I need this trackBy, otherwise, when I write inside input's, i lose the focus :-(.</p>
  
  <p><a href="https://stackblitz.com/edit/angular-o5jqug?file=src%2Fapp%2Fhello.component.ts" rel="nofollow noreferrer">my code simplified</a></p>
  
  <p>I tried when delete; filter but my objects is <code>FormArray</code> or <code>AbstractControl</code>[]</p>