Use Object.observe() instead of Polyfills

ui/file_manager/audio_player/elements/audio_player.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 Polymer('audio-player', {
   /**
    * Child Elements
    */
   audioController: null,
   audioElement: null,
@@ skipping 221 matching lines @@
 
     // If there is only a single file in the list, 'currentTrackInde' is not
     // changed and the handler is not invoked. Instead, plays here.
     // TODO(yoshiki): clean up the code around here.
     if (isNextTrackAvailable &&
         this.trackList.currentTrackIndex == nextTrackIndex) {
       this.audioElement.play();
     }
 
     this.trackList.currentTrackIndex = nextTrackIndex;
-
-    Platform.performMicrotaskCheckpoint();
   },
 
   /**
    * Timeout ID of auto advance. Used internally in scheduleAutoAdvance_() and
    *     cancelAutoAdvance_().
    * @type {number}
    * @private
    */
   autoAdvanceTimer_: null,
 
@@ skipping 117 matching lines @@
         break;
       case 'MediaPreviousTrack':
         this.onControllerPreviousClicked();
         break;
       case 'MediaStop':
         // TODO: Define "Stop" behavior.
         break;
     }
   },
 });