Roll Observatory packages and add a roll script

packages/web_components/lib/html_import_annotation.dart

 // Copyright (c) 2015, 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.
 library web_components.html_import_annotation;
 
 import 'dart:async';
 import 'dart:html';
 import 'package:initialize/initialize.dart';
 import 'src/normalize_path.dart';
 
@@ skipping 16 matching lines @@
       element.on['error'].listen((_) {
         print('Error loading html import from path `$filePath`');
         completer.complete();
       }),
     ];
     return completer.future.then((_) {
       listeners.forEach((listener) => listener.cancel());
     });
   }
 }