[MD Bookmarks] Change selection items from Map to Set.

chrome/test/data/webui/md_bookmarks/dnd_manager_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('drag and drop', function() {
   var app;
   var list;
   var sidebar;
   var store;
   var dndManager;
@@ skipping 215 matching lines @@
         DropPosition.ON | DropPosition.ABOVE,
         dndManager.calculateValidDropPositions_(dragTarget));
 
     dispatchDragEvent('dragover', dragTarget);
 
     assertDragStyle(dragTarget, DRAG_STYLE.ON);
 
     dispatchDragEvent('dragend', dragElement);
     assertDragStyle(dragTarget, DRAG_STYLE.NONE);
 
-    store.data.closedFolders['11'] = true;
+    store.data.closedFolders.add('11');
 
     dispatchDragEvent('dragstart', dragElement);
     dndManager.dragInfo_.handleChromeDragEnter(createDragData(draggedIds));
 
     // Drags below a closed folder are allowed.
     assertEquals(
         DropPosition.ON | DropPosition.ABOVE | DropPosition.BELOW,
         dndManager.calculateValidDropPositions_(dragTarget));
   });
 
   test('drag multiple list items', function() {
     // Dragging multiple items.
-    store.data.selection.items = {'13': true, '15': true};
+    store.data.selection.items = new Set(['13', '15']);
     dispatchDragEvent('dragstart', getListItem('13'));
     assertDeepEquals(['13', '15'], draggedIds);
     dndManager.dragInfo_.handleChromeDragEnter(createDragData(draggedIds));
 
     // The dragged items should not be allowed to be dragged around any selected
     // item.
     assertEquals(
         DropPosition.NONE,
         dndManager.calculateValidDropPositions_(getListItem('13')));
     assertEquals(
@@ skipping 120 matching lines @@
     assertDragStyle(dragTarget, DRAG_STYLE.ABOVE);
 
     dispatchDragEvent('drop', dragTarget);
     assertEquals('1', dropParentId);
     assertEquals(1, dropIndex);
 
     dispatchDragEvent('dragend', dragTarget);
     assertDragStyle(dragTarget, DRAG_STYLE.NONE);
   });
 });