required only after ng blur and ng change

gistfile1.html

<!-- parcialmente basado en yearofmoo.com  -->
<form name="formulario">
  <div class="field">
    <label for="correo">Correo Electronico:</label>
    <input type="email"
           name="correo" ng-change="MiCtrl.checkRequired(datos.email)" ng-blur="MiCtrl.checkRequired(datos.email)"
           ng-model="datos.email"
            />
    <div ng-messages="formulario.correo.$error">
      <div ng-message="required">Campo requerido</div>
      <div ng-message="email">Debe ser un correo electronico.</div>
    </div>
  </div>
</form>
    <!-- MiCtrl controller -->
    ...
  this.checkRequired = function(modelCtrl) {
    var isRequired = (modelCtrl.$viewValue || '').length === 0;
    modelCtrl.$error.required = isRequired;
    modelCtrl.$setValidity(modelCtrl.$name, !isRequired);
  };
  
  ...