Files.app: fix some closure compilation issues to unblock new compiler roll.

ui/file_manager/file_manager/foreground/js/directory_contents.js

 // Copyright (c) 2012 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.
 
 /**
  * Scanner of the entries.
  * @constructor
  */
 function ContentScanner() {
   this.cancelled_ = false;
@@ skipping 610 matching lines @@
   this.metadataSnapshot_ = null;
 }
 
 /**
  * DirectoryContents extends cr.EventTarget.
  */
 DirectoryContents.prototype.__proto__ = cr.EventTarget.prototype;
 
 /**
  * Create the copy of the object, but without scan started.
- * @return {DirectoryContents} Object copy.
+ * @return {!DirectoryContents} Object copy.
  */
 DirectoryContents.prototype.clone = function() {
   return new DirectoryContents(
       this.context_,
       this.isSearch_,
       this.directoryEntry_,
       this.scannerFactory_);
 };
 
 /**
@@ skipping 411 matching lines @@
 DirectoryContents.createForDriveMetadataSearch = function(
     context, fakeDirectoryEntry, searchType) {
   return new DirectoryContents(
       context,
       true,  // Search
       fakeDirectoryEntry,
       function() {
         return new DriveMetadataSearchContentScanner(searchType);
       });
 };