restarts a song when the next track button is clicked and there is only one song in audio player

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({
   is: 'audio-player',
 
   properties: {
     /**
      * Flag whether the audio is playing or paused. True if playing, or false
@@ skipping 253 matching lines @@
    */
   advance_: function(forward, repeat) {
     this.cancelAutoAdvance_();
 
     var nextTrackIndex = this.$.trackList.getNextTrackIndex(forward, true);
     var isNextTrackAvailable =
         (this.$.trackList.getNextTrackIndex(forward, repeat) !== -1);
 
     this.playing = isNextTrackAvailable;
 
-    // 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.$.trackList.currentTrackIndex == nextTrackIndex) {
-      this.$.audio.play();
+    var shouldFireEvent = this.$.trackList.currentTrackIndex === nextTrackIndex;
+    this.$.trackList.currentTrackIndex = nextTrackIndex;
+    this.$.audio.currentTime = 0;
+    // If the next track and current track is the same,
+    // the event will not be fired.
+    // So we will fire the event here.
+    // This happenes if there is only one song.
+    if (shouldFireEvent) {
+      this.$.trackList.fire('current-track-index-changed');
     }
-
-    this.$.trackList.currentTrackIndex = nextTrackIndex;
   },
 
   /**
    * Timeout ID of auto advance. Used internally in scheduleAutoAdvance_() and
    *     cancelAutoAdvance_().
    * @type {number?}
    * @private
    */
   autoAdvanceTimer_: null,
 
@@ skipping 144 matching lines @@
 
   /**
    * Computes volume value for audio element. (should be in [0.0, 1.0])
    * @param {number} volume Volume which is set in the UI. ([0, 100])
    * @return {number}
    */
   computeAudioVolume_: function(volume) {
     return volume / 100;
   }
 });