Roll Observatory packages and add a roll script

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);
-}