Roll Observatory packages and add a roll script

plugin/lib/manager.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 plugin.manager;
-
-import 'package:plugin/src/plugin_impl.dart';
-import 'package:plugin/plugin.dart';
-
-/**
- * An object that manages the extension points for the host application.
- */
-abstract class ExtensionManager {
-  /**
-   * Initialize a newly created manager.
-   */
-  factory ExtensionManager() = ExtensionManagerImpl;
-
-  /**
-   * Process each of the given [plugins] by allowing them to register extension
-   * points and extensions.
-   *
-   * An [ExtensionError] will be thrown if any of the plugins throws such an
-   * exception while registering with this manager.
-   */
-  void processPlugins(List<Plugin> plugins);
-}