[MD Bookmarks] Make dragging update selected items.

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 rootFolderNode;
   var store;
   var dndManager;
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     store.notifyObservers();
 
     dispatchDragEvent('dragstart', dragElement);
     dndManager.dragInfo_.handleChromeDragEnter(createDragData(draggedIds));
 
     dispatchDragEvent('dragover', dragTarget);
     assertEquals(
         DropPosition.NONE, dndManager.calculateValidDropPositions_(dragTarget));
     assertDragStyle(dragTarget, DRAG_STYLE.NONE);
   });
+
+  test('drag item selects/deselects items', function() {
+    store.setReducersEnabled(true);
+
+    store.data.selection.items = new Set(['13', '15']);
+    store.notifyObservers();
+
+    // Dragging an item not in the selection selects the dragged item and
+    // deselects the previous selection.
+    var dragElement = getListItem('14');
+    dispatchDragEvent('dragstart', dragElement);
+    assertDeepEquals(['14'], normalizeSet(store.data.selection.items));
+    dispatchDragEvent('dragend', dragElement);
+
+    // Dragging a folder node deselects any selected items in the bookmark list.
+    dragElement = getFolderNode('15');
+    dispatchDragEvent('dragstart', dragElement);
+    assertDeepEquals([], normalizeSet(store.data.selection.items));
+    dispatchDragEvent('dragend', dragElement);
+  });
 });