Use Object.observe() instead of Polyfills

ui/file_manager/audio_player/elements/track_list.js

Index: ui/file_manager/audio_player/elements/track_list.js
diff --git a/ui/file_manager/audio_player/elements/track_list.js b/ui/file_manager/audio_player/elements/track_list.js
index 353083d88bf56f5c53ccf59d7e12d5208c4e726d..69b9f90913af732c614de4b925a1dfa434ed7bfe 100644
--- a/ui/file_manager/audio_player/elements/track_list.js
+++ b/ui/file_manager/audio_player/elements/track_list.js
@@ -11,13 +11,26 @@
      * 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
      */
@@ -101,9 +114,7 @@
       // 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.
@@ -124,7 +135,7 @@
      * 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