Roll Observatory packages and add a roll script

packages/plugin/lib/src/plugin_impl.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.src.plugin_impl;
 
 import 'dart:collection';
 
 import 'package:plugin/manager.dart';
 import 'package:plugin/plugin.dart';
@@ skipping 99 matching lines @@
    * Validate that the given [extension] is appropriate for this extension
    * point, and if it is then add it to the list of registered exceptions.
    */
   void add(Object extension) {
     if (validateExtension != null) {
       validateExtension(extension);
     }
     _extensions.add(extension);
   }
 }