title: checkbox unchecked to make form invalid
body: <p>trying to add form validators for the checkbox in my form, in case if it's unchecked the form will have <code>ng-invalid</code>.</p>
  
  <pre><code>&lt;li class="terms_checkbox"&gt;&lt;input type="checkbox" id="checkbox_terms" name="terms" /&gt;&lt;label for="checkbox_terms"&gt;&lt;/label&gt;&lt;/li&gt;
  &lt;li class="accept"&gt;I have read and accept the terms and conditions&lt;/li&gt;
  </code></pre>
  
  <p>any advise on what can trigger the addition of <code>ng-invalid</code> to the form if it's unchecked and vice versa for the class removal. </p>