fix($animate): ensure transition animations are unblocked before the dom operation occurs

Transitions are blocked when the base CSS class is added at the start of the animation. This
causes an issue if the followup CSS class contains animatable-styles. Now, once the animation
active state is triggered (when the animation CSS dom operation occurs) the animation itself
will always trigger an animate without a quick jump.

Closes #5014
Closes #4265

test/ngAnimate/animateSpec.js

@@ -2663,4 +2663,37 @@ describe("ngAnimate", function() {
     expect(element.hasClass('base-class')).toBe(true);
   }));
 
+  it('should block and unblock transitions before the dom operation occurs',
+    inject(function($rootScope, $compile, $rootElement, $document, $animate, $sniffer, $timeout) {
+
+    if (!$sniffer.transitions) return;
+
+    $animate.enabled(true);
+
+    ss.addRule('.cross-animation', '-webkit-transition:1s linear all;' +
+                                           'transition:1s linear all;');
+
+    var capturedProperty = 'none';
+
+    var element = $compile('<div class="cross-animation"></div>')($rootScope);
+    $rootElement.append(element);
+    jqLite($document[0].body).append($rootElement);
+
+    var node = element[0];
+    node._setAttribute = node.setAttribute;
+    node.setAttribute = function(prop, val) {
+      if(prop == 'class' && val.indexOf('trigger-class') >= 0) {
+        var propertyKey = ($sniffer.vendorPrefix == 'Webkit' ? '-webkit-' : '') + 'transition-property';
+        capturedProperty = element.css(propertyKey);
+      }
+      node._setAttribute(prop, val);
+    };
+
+    $animate.addClass(element, 'trigger-class');
+
+    $timeout.flush();
+
+    expect(capturedProperty).not.toBe('none');
+  }));
+
 });