fix(animations): retain styling when transition destinations are changed (#12208)

Closes #9661
Closes #12208

modules/@angular/platform-browser/testing/mock_animation_driver.ts

@@ -9,19 +9,29 @@
 import {AnimationPlayer} from '@angular/core';
 import {MockAnimationPlayer} from '@angular/core/testing/testing_internal';
 import {AnimationDriver} from '@angular/platform-browser';
+
+import {ListWrapper} from './facade/collection';
 import {AnimationKeyframe, AnimationStyles} from './private_import_core';
 
 export class MockAnimationDriver extends AnimationDriver {
   public log: {[key: string]: any}[] = [];
   animate(
       element: any, startingStyles: AnimationStyles, keyframes: AnimationKeyframe[],
-      duration: number, delay: number, easing: string): AnimationPlayer {
-    const player = new MockAnimationPlayer();
+      duration: number, delay: number, easing: string,
+      previousPlayers: AnimationPlayer[] = []): AnimationPlayer {
+    const mockPlayers = <MockAnimationPlayer[]>previousPlayers.filter(
+        player => player instanceof MockAnimationPlayer);
+    const normalizedStartingStyles = _serializeStyles(startingStyles);
+    const normalizedKeyframes = _serializeKeyframes(keyframes);
+    const player =
+        new MockAnimationPlayer(normalizedStartingStyles, normalizedKeyframes, previousPlayers);
+
     this.log.push({
       'element': element,
-      'startingStyles': _serializeStyles(startingStyles),
+      'startingStyles': normalizedStartingStyles,
+      'previousStyles': player.previousStyles,
       'keyframes': keyframes,
-      'keyframeLookup': _serializeKeyframes(keyframes),
+      'keyframeLookup': normalizedKeyframes,
       'duration': duration,
       'delay': delay,
       'easing': easing,