feat(ngModelController): add $setDirty method

- extract existing functionality to public method: $setDirty
- add tests to corresponding changes
- refactor code to use extracted method

Closes #10038
Closes #10049

src/ng/directive/input.js

@@ -1870,6 +1870,25 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
     $animate.addClass($element, PRISTINE_CLASS);
   };
 
+  /**
+   * @ngdoc method
+   * @name ngModel.NgModelController#$setDirty
+   *
+   * @description
+   * Sets the control to its dirty state.
+   *
+   * This method can be called to remove the `ng-pristine` class and set the control to its dirty
+   * state (`ng-dirty` class). A model is considered to be dirty when the control has been changed
+   * from when first compiled.
+   */
+  this.$setDirty = function() {
+    ctrl.$dirty = true;
+    ctrl.$pristine = false;
+    $animate.removeClass($element, PRISTINE_CLASS);
+    $animate.addClass($element, DIRTY_CLASS);
+    parentForm.$setDirty();
+  };
+
   /**
    * @ngdoc method
    * @name ngModel.NgModelController#$setUntouched
@@ -2139,11 +2158,7 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
 
     // change to dirty
     if (ctrl.$pristine) {
-      ctrl.$dirty = true;
-      ctrl.$pristine = false;
-      $animate.removeClass($element, PRISTINE_CLASS);
-      $animate.addClass($element, DIRTY_CLASS);
-      parentForm.$setDirty();
+      this.$setDirty();
     }
     this.$$parseAndValidate();
   };

test/ng/directive/inputSpec.js

@@ -139,6 +139,23 @@ describe('NgModelController', function() {
     });
   });
 
+  describe('setDirty', function() {
+
+    it('should set control to its dirty state', function() {
+      expect(ctrl.$pristine).toBe(true);
+      expect(ctrl.$dirty).toBe(false);
+
+      ctrl.$setDirty();
+      expect(ctrl.$pristine).toBe(false);
+      expect(ctrl.$dirty).toBe(true);
+    });
+
+    it('should set parent form to its dirty state', function() {
+      ctrl.$setDirty();
+      expect(parentFormCtrl.$setDirty).toHaveBeenCalled();
+    });
+  });
+
   describe('setUntouched', function() {
 
     it('should set control to its untouched state', function() {