title: Form $validators without ng-model
body: <p>I have a <code>form</code> element:</p>
  
  <pre><code>&lt;form name="formDate"&gt;
      &lt;input date-picker name="fundacao" ng-model-date="fTit.fundacao" required&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>And the directive: </p>
  
  <pre><code>app.directive('datePicker', function(){
      return {
          restrict: 'A',
          scope: {
              ngModelDate: "=ngModelDate"
          },
          link: function (ng, el, attr) {     
              // $validators HERE
          }
      };
  })
  </code></pre>
  
  <p>The "required" validation does not work because there is no <code>ng-model</code> directive on the element.</p>
  
  <p>Is there any way to validate a form with nonstandard directives, such as the example above?</p>