Refactor DirectoryTree part 1 of 2

This is part of a reverted CL crrev.com/c/1300993, but this is the safe
part of it. I don't expect this to cause flakiness.

Change updateSubDirectories:
- use arrow function and const;
- change error callback calling style to match the same style used by
success callback (without if).
- change check from isFakeEntry to check for presence of the method
that's used "createReader".

Change the sorting to be an method, so it can be customized per
sub-class, later it will be used to sort My files to show Linux and Play
files at the bottom.

No changes in behaviour for users.

Change-Id: Ie8542464a360b5ed1970183dc888003119b423a9
Reviewed-on: https://chromium-review.googlesource.com/c/1313710
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604768}

ui/file_manager/file_manager/foreground/js/ui/directory_tree.js

@@ -482,6 +482,17 @@ DirectoryItem.prototype.handleClick = function(e) {
   }
 };
 
+/**
+ * Default sorting for DirectoryItem sub-dirrectories.
+ * @param {!Array<!Entry>} entries Entries to be sorted.
+ * @returns {!Array<!Entry>}
+ */
+DirectoryItem.prototype.sortEntries = function(entries) {
+  entries.sort(util.compareName);
+  const filter = this.fileFilter_.filter.bind(this.fileFilter_);
+  return entries.filter(filter);
+};
+
 /**
  * Retrieves the latest subdirectories and update them on the tree.
  * @param {boolean} recursive True if the update is recursively.
@@ -490,32 +501,23 @@ DirectoryItem.prototype.handleClick = function(e) {
  */
 DirectoryItem.prototype.updateSubDirectories = function(
     recursive, opt_successCallback, opt_errorCallback) {
-  if (!this.entry || util.isFakeEntry(this.entry)) {
-    if (opt_errorCallback)
-      opt_errorCallback();
+  if (!this.entry || this.entry.createReader === undefined) {
+    opt_errorCallback && opt_errorCallback();
     return;
   }
-
-  const sortEntries = (fileFilter, entries) => {
-    entries.sort(util.compareName);
-    return entries.filter(fileFilter.filter.bind(fileFilter));
-  };
-
   const onSuccess = (entries) => {
     this.entries_ = entries;
     this.updateSubElementsFromList(recursive);
     opt_successCallback && opt_successCallback();
   };
-
   const reader = this.entry.createReader();
   const entries = [];
   const readEntry = () => {
     reader.readEntries((results) => {
       if (!results.length) {
-        onSuccess(sortEntries(this.fileFilter_, entries));
+        onSuccess(this.sortEntries(entries));
         return;
       }
-
       for (let i = 0; i < results.length; i++) {
         const entry = results[i];
         if (entry.isDirectory)