Roll Observatory packages and add a roll script

packages/web_components/lib/src/custom_element.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;
 
 import 'dart:html' show document;
 import 'package:initialize/initialize.dart';
 
 /// Annotation which registers a custom element with the main document.
 class CustomElement implements Initializer<Type> {
   /// The tag you want to register the class to handle.
   final String tag;
 
   /// If this element extends a native html element, then this is the tag that
   /// represents that element.
   final String extendsTag;
 
   const CustomElement(this.tag, {this.extendsTag});
 
   void initialize(Type t) =>
       document.registerElement(tag, t, extendsTag: extendsTag);
 }