[MD Bookmarks] Prevent sorting of empty folders.

chrome/test/data/webui/md_bookmarks/toolbar_test.js

 // Copyright 2017 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.
 
 suite('<bookmarks-toolbar>', function() {
   var toolbar;
   var store;
   var commandManager;
 
   suiteSetup(function() {
     chrome.bookmarkManagerPrivate.removeTrees = function() {};
   });
 
   setup(function() {
     store = new bookmarks.TestStore({
       nodes: testTree(createFolder(
           '1',
           [
             createItem('2'),
             createItem('3'),
             createFolder('4', [], {unmodifiable: 'managed'}),
+            createFolder('5', []),
+            createFolder(
+                '6',
+                [
+                  createItem('61'),
+                ]),
           ])),
       selection: {
         items: new Set(),
         anchor: null,
       },
     });
     store.replaceSingleton();
 
     toolbar = document.createElement('bookmarks-toolbar');
     replaceBody(toolbar);
@@ skipping 69 matching lines @@
 
     assertTrue(toolbar.$$('#addBookmarkButton').disabled);
     assertFalse(toolbar.$$('#importBookmarkButton').disabled);
 
     store.data.prefs.canEdit = false;
     store.notifyObservers();
 
     assertTrue(toolbar.$$('#addBookmarkButton').disabled);
     assertTrue(toolbar.$$('#importBookmarkButton').disabled);
   });
+
+  test('sort button is disabled when folder is empty', function() {
+    MockInteractions.tap(toolbar.$.menuButton);
+
+    store.data.selectedFolder = '6';
+    store.notifyObservers();
+    Polymer.dom.flush();

[tsergeant, 2017/07/10 07:09:08]

Nit: Are these flushes actually necessary? I wouldn't really expect them to be,
so remove them if you can.

[calamity, 2017/07/11 06:53:10]

Done.

+    assertTrue(toolbar.canSortFolder_);
+
+    store.data.selectedFolder = '5';
+    store.notifyObservers();
+    Polymer.dom.flush();
+
+    assertFalse(toolbar.canSortFolder_);
+    assertTrue(toolbar.$$('#sortButton').disabled);
+  });
 });