Fix all compiler errors found by the new compiler version.

ui/file_manager/file_manager/background/js/device_handler.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.
 
 /**
  * Handler of device event.
  * @constructor
  * @extends {cr.EventTarget}
  */
 function DeviceHandler() {
@@ skipping 405 matching lines @@
           function(volumeManager) {
             if (!metadata.volumeId) {
               return Promise.reject('No volume id associated with event.');
             }
             return volumeManager.volumeInfoList.whenVolumeInfoReady(
                 metadata.volumeId);
           })
       .then(
           /**
            * @param {!VolumeInfo} volumeInfo
-           * @return {Promise<DirectoryEntry>} The root directory
+           * @return {!Promise<!DirectoryEntry>} The root directory
            *     of the volume.
            */
           function(volumeInfo) {
             return importer.importEnabled()
                 .then(
                     /** @param {boolean} enabled */
                     function(enabled) {
                       if (enabled && importer.isEligibleVolume(volumeInfo)) {
                         return volumeInfo.resolveDisplayRoot();
                       }
@@ skipping 99 matching lines @@
  * @param {string} volumeId
  * @param {string} path
  * @private
  */
 DeviceHandler.prototype.openMediaDirectory_ = function(volumeId, path) {
   var event = new Event(DeviceHandler.VOLUME_NAVIGATION_REQUESTED);
   event.volumeId = volumeId;
   event.filePath = path;
   this.dispatchEvent(event);
 };