Use Object.observe() instead of Polyfills

ui/file_manager/audio_player/elements/track_list.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.
 
 (function() {
   'use strict';
 
   Polymer('track-list', {
     /**
      * Initializes an element. This method is called automatically when the
      * element is ready.
      */
     ready: function() {
-      this.tracksObserver_ = new ArrayObserver(
-          this.tracks,
-          this.tracksValueChanged_.bind(this));
+      this.observeTrackList();
 
       window.addEventListener('resize', this.onWindowResize_.bind(this));
     },
 
+    observeTrackList: function() {
+      // Unobserve the previous track list.
+      if (this.unobserveTrackList_)
+        this.unobserveTrackList_();
+
+      // Observe the new track list.
+      var observer = this.tracksValueChanged_.bind(this);
+      Array.observe(this.tracks, observer);
+
+      // Set the function to unobserve it.
+      this.unobserveTrackList_ = function(tracks, observer) {
+        Array.unobserve(tracks, observer);
+      }.bind(null, this.tracks, observer);
+    },
+
     /**
      * Registers handlers for changing of external variables
      */
     observe: {
       'model.shuffle': 'onShuffleChanged',
     },
 
     /**
      * Model object of the Audio Player.
      * @type {AudioPlayerModel}
@@ skipping 63 matching lines @@
     /**
      * Invoked when 'tracks' property is changed.
      * @param {Array.<TrackInfo>} oldValue Old value.
      * @param {Array.<TrackInfo>} newValue New value.
      */
     tracksChanged: function(oldValue, newValue) {
       // Note: Sometimes both oldValue and newValue are null though the actual
       // values are not null. Maybe it's a bug of Polymer.
 
       // Re-register the observer of 'this.tracks'.
-      this.tracksObserver_.close();
-      this.tracksObserver_ = new ArrayObserver(this.tracks);
-      this.tracksObserver_.open(this.tracksValueChanged_.bind(this));
+      this.observeTrackList();
 
       if (this.tracks.length !== 0) {
         // Restore the active track.
         if (this.currentTrackIndex !== -1 &&
             this.currentTrackIndex < this.tracks.length) {
           this.tracks[this.currentTrackIndex].active = true;
         }
 
         // Reset play order and current index.
         this.generatePlayOrder(false /* no need to keep the current track */);
       } else {
         this.playOrder = [];
         this.currentTrackIndex = -1;
       }
     },
 
     /**
      * Invoked when the value in the 'tracks' is changed.
      * @param {Array.<Object>} splices The detail of the change.

[fukino, 2015/03/06 12:14:21]

nit: splices -> changes in annotation

      */
-    tracksValueChanged_: function(splices) {
+    tracksValueChanged_: function(changes) {
       if (this.tracks.length === 0)
         this.currentTrackIndex = -1;
       else
         this.tracks[this.currentTrackIndex].active = true;
     },
 
     /**
      * Invoked when the track element is clicked.
      * @param {Event} event Click event.
      */
@@ skipping 150 matching lines @@
 
       var newTrackIndex = this.playOrder[newPlayOrder];
       console.assert(
           (0 <= newTrackIndex && newTrackIndex < this.tracks.length),
           'Insufficient TrackList.playOrder. New Play Order: ' + newPlayOrder);
 
       return newTrackIndex;
     },
   });  // Polymer('track-list') block
 })();  // Anonymous closure