fix(modal): switch to $animate

- Change to adding backdrop with $animate.addClass due to $animateCss
  issues when animation is disabled

Closes #5585
Fixes #5298

src/modal/modal.js

@@ -103,8 +103,8 @@ angular.module('ui.bootstrap.modal', ['ui.bootstrap.stackedMap'])
 /**
  * A helper directive for the $modal service. It creates a backdrop element.
  */
-  .directive('uibModalBackdrop', ['$animateCss', '$injector', '$uibModalStack',
-  function($animateCss, $injector, $modalStack) {
+  .directive('uibModalBackdrop', ['$animate', '$injector', '$uibModalStack',
+  function($animate, $injector, $modalStack) {
     return {
       replace: true,
       templateUrl: 'uib/template/modal/backdrop.html',
@@ -116,16 +116,12 @@ angular.module('ui.bootstrap.modal', ['ui.bootstrap.stackedMap'])
 
     function linkFn(scope, element, attrs) {
       if (attrs.modalInClass) {
-        $animateCss(element, {
-          addClass: attrs.modalInClass
-        }).start();
+        $animate.addClass(element, attrs.modalInClass);
 
         scope.$on($modalStack.NOW_CLOSING_EVENT, function(e, setIsAsync) {
           var done = setIsAsync();
           if (scope.modalOptions.animation) {
-            $animateCss(element, {
-              removeClass: attrs.modalInClass
-            }).start().then(done);
+            $animate.removeClass(element, attrs.modalInClass).then(done);
           } else {
             done();
           }