fix($animate): ensure $animate doesn't break natural CSS transitions

BREAKING CHANGE: ngClass and {{ class }} will now call the `setClass`
animation callback instead of addClass / removeClass when both a
addClass/removeClass operation is being executed on the element during the animation.

Please include the setClass animation callback as well as addClass and removeClass within
your JS animations to work with ngClass and {{ class }} directives.

Closes #6019

test/ng/compileSpec.js

@@ -4666,11 +4666,9 @@ describe('$compile', function() {
         $rootScope.$digest();
 
         data = $animate.queue.shift();
-        expect(data.event).toBe('removeClass');
-        expect(data.args[1]).toBe('rice');
-        data = $animate.queue.shift();
-        expect(data.event).toBe('addClass');
+        expect(data.event).toBe('setClass');
         expect(data.args[1]).toBe('dice');
+        expect(data.args[2]).toBe('rice');
 
         expect(element.hasClass('ice')).toBe(true);
         expect(element.hasClass('dice')).toBe(true);

test/ng/directive/ngClassSpec.js

@@ -335,8 +335,7 @@ describe('ngClass animations', function() {
 
       $rootScope.val = 'two';
       $rootScope.$digest();
-      expect($animate.queue.shift().event).toBe('removeClass');
-      expect($animate.queue.shift().event).toBe('addClass');
+      expect($animate.queue.shift().event).toBe('setClass');
       expect($animate.queue.length).toBe(0);
     });
   });
@@ -450,12 +449,9 @@ describe('ngClass animations', function() {
       $rootScope.$digest();
 
       item = $animate.queue.shift();
-      expect(item.event).toBe('removeClass');
-      expect(item.args[1]).toBe('two');
-
-      item = $animate.queue.shift();
-      expect(item.event).toBe('addClass');
+      expect(item.event).toBe('setClass');
       expect(item.args[1]).toBe('three');
+      expect(item.args[2]).toBe('two');
 
       expect($animate.queue.length).toBe(0);
     });