feat(v2): blog support date front matter (#1702)

* feat(v2): blog support date front matter

* feat(v2): blog support date front matter

* feat(v2): blog support date front matter

* feat(v2): blog support date front matter

* Update CHANGELOG-2.x.md

packages/docusaurus-plugin-content-blog/src/__tests__/index.test.js

@@ -0,0 +1,76 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import fs from 'fs-extra';
+import path from 'path';
+import pluginContentBlog from '../index';
+
+describe('loadBlog', () => {
+  test('simple website', async () => {
+    const siteDir = path.join(__dirname, '__fixtures__', 'website');
+    const siteConfig = {
+      title: 'Hello',
+      baseUrl: '/',
+      url: 'https://docusaurus.io',
+    };
+    const pluginPath = 'blog';
+    const plugin = pluginContentBlog(
+      {
+        siteDir,
+        siteConfig,
+      },
+      {
+        path: 'blog',
+      },
+    );
+    const {blogPosts} = await plugin.loadContent();
+
+    expect(
+      blogPosts.find(v => v.metadata.title === 'date-matter').metadata,
+    ).toEqual({
+      permalink: '/blog/2019/01/01/date-matter',
+      source: path.join('@site', pluginPath, 'date-matter.md'),
+      title: 'date-matter',
+      description: `date inside front matter`,
+      date: new Date('2019-01-01'),
+      tags: [],
+    });
+    expect(
+      blogPosts.find(v => v.metadata.title === 'Happy 1st Birthday Slash!')
+        .metadata,
+    ).toEqual({
+      permalink: '/blog/2018/12/14/Happy-First-Birthday-Slash',
+      source: path.join(
+        '@site',
+        pluginPath,
+        '2018-12-14-Happy-First-Birthday-Slash.md',
+      ),
+      title: 'Happy 1st Birthday Slash!',
+      description: `pattern name`,
+      date: new Date('2018-12-14'),
+      tags: [],
+    });
+
+    const noDateSource = path.join('@site', pluginPath, 'no date.md');
+    const noDateSourceBirthTime = (await fs.stat(
+      noDateSource.replace('@site', siteDir),
+    )).birthtime;
+    expect(
+      blogPosts.find(v => v.metadata.title === 'no date').metadata,
+    ).toEqual({
+      permalink: `/blog/${noDateSourceBirthTime
+        .toISOString()
+        .substr(0, '2019-01-01'.length)
+        .replace(/-/g, '/')}/no date`,
+      source: noDateSource,
+      title: 'no date',
+      description: `no date`,
+      date: noDateSourceBirthTime,
+      tags: [],
+    });
+  });
+});