Add (failing) source_update_test.dart.

dart/tests/try/mock_try.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:html' show
+    DivElement,
+    MutationObserver,
+    document;
+
+import '../../site/try/src/interaction_manager.dart' show
+    InteractionManager;
+
+import '../../site/try/src/ui.dart' show
+    hackDiv,
+    mainEditorPane,
+    observer;
+
+import '../../site/try/src/user_option.dart' show
+    UserOption;
+
+InteractionManager mockTryDartInteraction() {
+  UserOption.storage = {};
+
+  InteractionManager interaction = new InteractionManager();
+
+  hackDiv = new DivElement();
+  mainEditorPane = new DivElement()
+      ..style.whiteSpace = 'pre'
+      ..contentEditable = 'true';
+
+  observer = new MutationObserver(interaction.onMutation);
+  observer.observe(
+      mainEditorPane, childList: true, characterData: true, subtree: true);
+
+  document.body.nodes.addAll([mainEditorPane, hackDiv]);
+
+  return interaction;
+}
+
+void clearEditorPaneWithoutNotifications() {
+  mainEditorPane.nodes.clear();
+  observer.takeRecords();
+}