Re-apply change to move to plugin package

pkg/analysis_server/lib/plugin/completion.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.
 
 /**
  * Support for client code that extends the analysis server by adding new code
  * completion contributors.
  */
 library analysis_server.plugin.completion;
 
 import 'package:analysis_server/completion/completion_core.dart';
 import 'package:analysis_server/src/plugin/server_plugin.dart';
-import 'package:analyzer/plugin/plugin.dart';
+import 'package:plugin/plugin.dart';
 
 /**
  * The identifier of the extension point that allows plugins to register new
  * code completion contributors with the server. The object used as an extension
  * must be a [CompletionContributor].
  */
 final String COMPLETION_CONTRIBUTOR_EXTENSION_POINT_ID = Plugin.join(
     ServerPlugin.UNIQUE_IDENTIFIER,
     ServerPlugin.COMPLETION_CONTRIBUTOR_EXTENSION_POINT);