Roll Observatory packages and add a roll script

packages/web_components/lib/custom_element_proxy.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.custom_element_proxy;
 
 import 'dart:js' as js;
 import 'package:initialize/initialize.dart';
 import 'interop.dart';
 
 /// Annotation for a dart class which proxies a javascript custom element.
@@ skipping 19 matching lines @@
 class CustomElementProxyMixin {
   js.JsObject _proxy;
 
   js.JsObject get jsElement {
     if (_proxy == null) {
       _proxy = new js.JsObject.fromBrowserObject(this);
     }
     return _proxy;
   }
 }