fix(modal): animate backdrop concurrently with window

- Start animation of backdrop at the same time as the window

Closes #4039
Closes #4036

Author: wesleycho

src/modal/modal.js

@@ -230,8 +230,8 @@ angular.module('ui.bootstrap.modal', [])
 
         removeAfterAnimate(modalWindow.modalDomEl, modalWindow.modalScope, function() {
           body.toggleClass(OPENED_MODAL_CLASS, openedWindows.length() > 0);
-          checkRemoveBackdrop();
         });
+        checkRemoveBackdrop();
 
         //move focus to specified element if available, or else to body
         if (elementToReceiveFocus && elementToReceiveFocus.focus) {

src/modal/test/modal.spec.js

@@ -179,7 +179,6 @@ describe('$modal', function () {
       expect($document).toHaveBackdrop();
 
       dismiss(modal, 'closing in test');
-      $animate.triggerCallbacks();
 
       expect($document).toHaveModalsOpen(0);
 
@@ -226,7 +225,6 @@ describe('$modal', function () {
       expect($document).toHaveBackdrop();
 
       dismiss(modal, 'closing in test');
-      $animate.triggerCallbacks();
 
       expect($document).toHaveModalsOpen(0);