Added custom tags helper classes to the Observatory

runtime/observatory/lib/elements.dart

 library observatory_elements;
 
 // Export elements.
 export 'package:observatory/src/elements/action_link.dart';
 export 'package:observatory/src/elements/class_ref.dart';
 export 'package:observatory/src/elements/class_tree.dart';
 export 'package:observatory/src/elements/class_view.dart';
 export 'package:observatory/src/elements/code_ref.dart';
 export 'package:observatory/src/elements/code_view.dart';
 export 'package:observatory/src/elements/context_ref.dart';
@@ skipping 41 matching lines @@
 export 'package:observatory/src/elements/script_ref.dart';
 export 'package:observatory/src/elements/script_view.dart';
 export 'package:observatory/src/elements/service_ref.dart';
 export 'package:observatory/src/elements/service_view.dart';
 export 'package:observatory/src/elements/sliding_checkbox.dart';
 export 'package:observatory/src/elements/timeline_page.dart';
 export 'package:observatory/src/elements/view_footer.dart';
 export 'package:observatory/src/elements/vm_connect.dart';
 export 'package:observatory/src/elements/vm_ref.dart';
 export 'package:observatory/src/elements/vm_view.dart';
+
+import 'dart:async';
+
+export 'package:observatory/src/elements/helpers/rendering_queue.dart';
+
+// Even though this function does not invoke any asynchronous operation
+// it is marked as async to allow future backward compatible changes.
+Future initElements() async {
+}