[MD Bookmarks] Make shift-selection behave well with no anchor.

chrome/test/data/webui/md_bookmarks/md_bookmarks_focus_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.
 
 /**
  * @fileoverview Tests for MD Bookmarks which are run as interactive ui tests.
  * Should be used for tests which care about focus.
  */
 
 var ROOT_PATH = '../../../../../';
@@ skipping 247 matching lines @@
       keydown(focusedItem, 'Escape');
       assertDeepEquals([], normalizeSet(store.data.selection.items));
 
       keydown(focusedItem, 'a', 'ctrl');
       assertDeepEquals(
           ['2', '3', '4', '5', '6', '7'],
           normalizeSet(store.data.selection.items));
     });
 
     test('shift selection', function() {
-      // TODO(calamity): Make the first item the anchor index when a new folder
-      // is selected.
       var focusedItem = items[0];
       focusedItem.focus();
 
-      keydown(focusedItem, 'ArrowDown');
+      keydown(focusedItem, 'ArrowDown', 'shift');
       focusedItem = items[1];
-      assertDeepEquals(['3'], normalizeSet(store.data.selection.items));
+      assertDeepEquals(['2', '3'], normalizeSet(store.data.selection.items));
+
+      keydown(focusedItem, 'Escape');
+      focusedItem = items[1];
+      assertDeepEquals([], normalizeSet(store.data.selection.items));
 
       keydown(focusedItem, 'ArrowUp', 'shift');
       focusedItem = items[0];
       assertDeepEquals(['2', '3'], normalizeSet(store.data.selection.items));
 
       keydown(focusedItem, 'ArrowDown', 'shift');
       focusedItem = items[1];
       assertDeepEquals(['3'], normalizeSet(store.data.selection.items));
 
       keydown(focusedItem, 'ArrowDown', 'shift');
@@ skipping 55 matching lines @@
 
       keydown(focusedItem, 'ArrowDown', ['ctrl', 'shift']);
       focusedItem = items[3];
       assertDeepEquals(
           ['2', '4', '5', '6'], normalizeSet(store.data.selection.items));
     });
   });
 
   mocha.run();
 });