Roll Observatory packages and add a roll script

packages/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);
 }