fix(animations): implement getPosition in browser animation builder

Forward `getPosition` to `animation_group_player`.

packages/animations/src/players/animation_group_player.ts

@@ -148,12 +148,13 @@ export class AnimationGroupPlayer implements AnimationPlayer {
   }
 
   getPosition(): number {
-    let min = 0;
-    this.players.forEach(player => {
-      const p = player.getPosition();
-      min = Math.min(p, min);
-    });
-    return min;
+    const longestPlayer =
+        this.players.reduce((longestSoFar: AnimationPlayer|undefined, player: AnimationPlayer) => {
+          const newPlayerIsLongest =
+              longestSoFar === undefined || player.totalTime > longestSoFar!.totalTime;
+          return newPlayerIsLongest ? player : longestSoFar;
+        }, undefined);
+    return longestPlayer != undefined ? longestPlayer.getPosition() / longestPlayer.totalTime : 0;
   }
 
   beforeDestroy(): void {

packages/animations/test/animation_group_player_spec.ts

@@ -0,0 +1,42 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+import {fakeAsync} from '@angular/core/testing';
+import {NoopAnimationPlayer} from '../src/animations';
+import {AnimationGroupPlayer} from '../src/players/animation_group_player';
+
+{
+  describe('AnimationGroupPlayer', function() {
+    it('should getPosition of an empty group', fakeAsync(() => {
+         const players: NoopAnimationPlayer[] = [];
+         const groupPlayer = new AnimationGroupPlayer(players);
+         expect(groupPlayer.getPosition()).toBe(0);
+       }));
+
+    it('should getPosition of a single player in a group', fakeAsync(() => {
+         const player = new NoopAnimationPlayer(5, 5);
+         player.getPosition = () => 2;
+         const players = [player];
+         const groupPlayer = new AnimationGroupPlayer(players);
+         expect(groupPlayer.getPosition()).toBe(0.2);
+       }));
+
+    it('should getPosition based on the longest player in the group', fakeAsync(() => {
+         const longestPlayer = new NoopAnimationPlayer(5, 5);
+         longestPlayer.getPosition = () => 2;
+         const players = [
+           new NoopAnimationPlayer(1, 4),
+           new NoopAnimationPlayer(4, 1),
+           new NoopAnimationPlayer(7, 0),
+           longestPlayer,
+           new NoopAnimationPlayer(1, 1),
+         ];
+         const groupPlayer = new AnimationGroupPlayer(players);
+         expect(groupPlayer.getPosition()).toBe(0.2);
+       }));
+  });
+}

packages/platform-browser/animations/src/animation_builder.ts

@@ -111,7 +111,7 @@ export class RendererAnimationPlayer implements AnimationPlayer {
   }
 
   getPosition(): number {
-    return 0;
+    return this._renderer.engine.players[+this.id].getPosition();
   }
 
   public totalTime = 0;