title: set invalid invalid form control in angular2
body: <p>Is it possible to set as invalid form control in angular2? (used form builder)</p>
  
  <blockquote>
    <p>eg. i have form-> exampleFrom  &amp; field-> exampleControl</p>
  </blockquote>
  
  <p>I have tried this, not succeed : </p>
  
  <pre><code>this.exampleFrom.controls['exampleControl'].invalid 
  </code></pre>