refactor(animations): ensure animation data-structures are created only when used

Closes #12250

modules/@angular/core/test/animation/active_animations_players_map_spec.ts

@@ -36,17 +36,17 @@ export function main() {
       expect(playersMap.find(elementNode, animationName)).toBe(player);
       expect(playersMap.findAllPlayersByElement(elementNode)).toEqual([player]);
       expect(playersMap.getAllPlayers()).toEqual([player]);
-      expect(playersMap.length).toEqual(1);
+      expect(countPlayers(playersMap)).toEqual(1);
     });
 
     it('should remove a registered player when remove() is called', () => {
       var player = new MockAnimationPlayer();
       playersMap.set(elementNode, animationName, player);
       expect(playersMap.find(elementNode, animationName)).toBe(player);
-      expect(playersMap.length).toEqual(1);
+      expect(countPlayers(playersMap)).toEqual(1);
       playersMap.remove(elementNode, animationName);
       expect(playersMap.find(elementNode, animationName)).not.toBe(player);
-      expect(playersMap.length).toEqual(0);
+      expect(countPlayers(playersMap)).toEqual(0);
     });
 
     it('should allow multiple players to be registered on the same element', () => {
@@ -54,7 +54,7 @@ export function main() {
       var player2 = new MockAnimationPlayer();
       playersMap.set(elementNode, 'myAnimation1', player1);
       playersMap.set(elementNode, 'myAnimation2', player2);
-      expect(playersMap.length).toEqual(2);
+      expect(countPlayers(playersMap)).toEqual(2);
       expect(playersMap.findAllPlayersByElement(elementNode)).toEqual([player1, player2]);
     });
 
@@ -63,10 +63,14 @@ export function main() {
       var player2 = new MockAnimationPlayer();
       playersMap.set(elementNode, animationName, player1);
       expect(playersMap.find(elementNode, animationName)).toBe(player1);
-      expect(playersMap.length).toEqual(1);
+      expect(countPlayers(playersMap)).toEqual(1);
       playersMap.set(elementNode, animationName, player2);
       expect(playersMap.find(elementNode, animationName)).toBe(player2);
-      expect(playersMap.length).toEqual(1);
+      expect(countPlayers(playersMap)).toEqual(1);
     });
   });
 }
+
+function countPlayers(map: ViewAnimationMap): number {
+  return map.getAllPlayers().length;
+}