[MD Bookmarks] Persist collapsed folders between page loads.

chrome/test/data/webui/md_bookmarks/app_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('app', function() {

[tsergeant, 2017/03/30 04:19:57]

Nit: Name the suite with the full element name: <bookmarks-app>

[calamity, 2017/04/03 03:48:41]

Done.

+  var app;
+  var store;
+
+  setup(function() {
+    store = new bookmarks.TestStore({
+      nodes: testTree(createFolder(
+          '1',
+          [
+            createFolder('11', []),
+          ])),
+      selectedFolder: '1',
+    });
+    bookmarks.Store.instance_ = store;
+    window.localStorage.clear();
+
+    app = document.createElement('bookmarks-app');
+    replaceBody(app);
+  });
+
+  test('write and load closed folder state', function() {
+    var closedFolders = {'1': true};
+    store.data.closedFolders = closedFolders;
+    store.notifyObservers();
+
+    // Ensure closed folders are written to local storage.
+    assertDeepEquals(
+        JSON.stringify(closedFolders),
+        window.localStorage[LOCAL_STORAGE_CLOSED_FOLDERS_KEY]);
+
+    store = new bookmarks.TestStore({});

[tsergeant, 2017/03/30 04:19:57]

I'm not keen on duplicating all the initialization stuff again here.

How about you pull this out into a reset() function, then call reset() from
setup() and from here. This version of the initialization code does some extra
stuff (acceptInit), but it doesn't really matter if that runs for the first part
of the test as well.

[calamity, 2017/04/03 03:48:41]

Done. I also changed acceptInit to acceptInitOnce so that we won't run into
bookmarks.getTree coming back at an inopportune time and ruining our TestStore.

+    store.acceptInit();
+    bookmarks.Store.instance_ = store;
+
+    chrome.bookmarks.getTree = function(fn) {
+      fn([
+        createFolder(
+            '0',
+            [
+              createFolder(
+                  '1',
+                  [
+                    createFolder('11', []),
+                  ]),
+            ]),
+      ]);
+    };
+    app = document.createElement('bookmarks-app');
+    replaceBody(app);
+
+    // Ensure closed folders are read from local storage.
+    assertDeepEquals(closedFolders, store.data.closedFolders);
+  });
+});