MD History: Add lazy-render element for simple lazy initialization.

chrome/test/data/webui/md_history/lazy_render_test.js

+// Copyright 2016 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.
+
+cr.define('md_history.lazy_render_test', function() {
+  function registerTests() {
+    suite('history-lazy-render', function() {
+
+      setup(function() {
+        PolymerTest.clearBody();
+        var template = '<template is="history-lazy-render" id="lazy">' +
+            '<h1>Hello world!</h1>' +
+            '</template>';
+
+        document.body.innerHTML = template;
+      });
+
+      test('stamps after get()', function() {
+        var lazy = document.getElementById('lazy');
+
+        assertFalse(!!document.body.querySelector('h1'));
+        assertFalse(!!lazy.getIfExists());
+
+        return lazy.get().then(function(inner) {
+          assertEquals('H1', inner.nodeName);
+          assertEquals(inner, document.body.querySelector('h1'));
+        });
+      });
+    });

[calamity, 2016/08/12 06:25:04]

Any way to ensure path forwarding and notifying works as expected?

[tsergeant, 2016/08/16 03:49:06]

Done.

[calamity, 2016/08/16 07:00:01]

Awesome!

+  }
+
+  return {
+    registerTests: registerTests,
+  }
+});