@ngdoc method
@name form.FormController#$setPristine

@description
Sets the form to its pristine state.

This method sets the form's `$pristine` state to true, the `$dirty` state to false, removes
the `ng-dirty` class and adds the `ng-pristine` class. Additionally, it sets the `$submitted`
state to false.

This method will also propagate to all the controls contained in this form.

Setting a form back to a pristine state is often useful when we want to 'reuse' a form after
saving or resetting it.
function() {
    this.$$animate.setClass(this.$$element, PRISTINE_CLASS, DIRTY_CLASS + ' ' + SUBMITTED_CLASS);
    this.$dirty = false;
    this.$pristine = true;
    this.$submitted = false;
    forEach(this.$$controls, function(control) {
      control.$setPristine();
    });
  }