feat(ngAnimate): add ng-[event]-prepare class for structural animations

The new prepare class is added before the animation is pushed to the
queue and removed before the animation runs. The class can be used to apply
CSS to explicitly hide these elements to prevent a flash of content
before the animation runs. This can happen if a structural animation
(such as ng-if) sits at the bottom of a tree which has ng-class animations
on the parents. Because child animations are spaced out with requestAnimationFrame,
the ng-enter class might not be applied in time, so the ng.if element is
briefly visible before its animation starts.

Author: matsko

src/ngAnimate/.jshintrc

@@ -28,6 +28,7 @@
     "REMOVE_CLASS_SUFFIX": false,
     "EVENT_CLASS_PREFIX": false,
     "ACTIVE_CLASS_SUFFIX": false,
+    "PREPARE_CLASS_SUFFIX": false,
 
     "TRANSITION_DURATION_PROP": false,
     "TRANSITION_DELAY_PROP": false,

src/ngAnimate/animation.js

@@ -135,6 +135,12 @@ var $$AnimationProvider = ['$animateProvider', function($animateProvider) {
         options.tempClasses = null;
       }
 
+      var prepareClassName;
+      if (isStructural) {
+        prepareClassName = 'ng-' + event + PREPARE_CLASS_SUFFIX;
+        $$jqLite.addClass(element, prepareClassName);
+      }
+
       animationQueue.push({
         // this data is used by the postDigest code and passed into
         // the driver step function
@@ -357,6 +363,10 @@ var $$AnimationProvider = ['$animateProvider', function($animateProvider) {
         if (tempClasses) {
           $$jqLite.addClass(element, tempClasses);
         }
+        if (prepareClassName) {
+          $$jqLite.removeClass(element, prepareClassName);
+          prepareClassName = null;
+        }
       }
 
       function updateAnimationRunners(animation, newRunner) {

src/ngAnimate/shared.js

@@ -20,6 +20,7 @@ var ADD_CLASS_SUFFIX = '-add';
 var REMOVE_CLASS_SUFFIX = '-remove';
 var EVENT_CLASS_PREFIX = 'ng-';
 var ACTIVE_CLASS_SUFFIX = '-active';
+var PREPARE_CLASS_SUFFIX = '-prepare';
 
 var NG_ANIMATE_CLASSNAME = 'ng-animate';
 var NG_ANIMATE_CHILDREN_DATA = '$$ngAnimateChildren';