Added custom tags helper classes to the Observatory

runtime/observatory/web/main.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 'package:logging/logging.dart';
 import 'package:polymer/polymer.dart';
+import 'package:observatory/elements.dart';
 
-main() {
+main() async {
   Logger.root.level = Level.INFO;
   Logger.root.onRecord.listen((LogRecord rec) {
       if (rec.level == Level.WARNING &&
           rec.message.startsWith('Error evaluating expression') &&
           (rec.message.contains("Can't assign to null: ") ||
            rec.message.contains('Expression is not assignable: '))) {
         // Suppress flaky polymer errors.
         return;
       }
       print('${rec.level.name}: ${rec.time}: ${rec.message}');
   });
+  await initElements();
   Logger.root.info('Starting Observatory');
-  initPolymer().then((zone) {
-    Logger.root.info('Polymer initialized');
-    // Code here is in the polymer Zone, which ensures that
-    // @observable properties work correctly.
-    Polymer.onReady.then((_) {
-      Logger.root.info('Polymer elements have been upgraded');
-    });
-  });
+  await initPolymer();
+  Logger.root.info('Polymer initialized');
+  await Polymer.onReady;
+  Logger.root.info('Polymer elements have been upgraded');
 }