Remove more references to the old plugin system

pkg/analysis_server/lib/src/socket_server.dart

 // Copyright (c) 2014, 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.
 
 import 'package:analysis_server/protocol/protocol.dart';
 import 'package:analysis_server/protocol/protocol_generated.dart';
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/channel/channel.dart';
 import 'package:analysis_server/src/server/diagnostic_server.dart';
 import 'package:analyzer/file_system/physical_file_system.dart';
 import 'package:analyzer/instrumentation/instrumentation.dart';
 import 'package:analyzer/plugin/resolver_provider.dart';
 import 'package:analyzer/source/pub_package_map_provider.dart';
 import 'package:analyzer/src/generated/sdk.dart';
-import 'package:plugin/plugin.dart';
 
 /**
  * Instances of the class [SocketServer] implement the common parts of
  * http-based and stdio-based analysis servers.  The primary responsibility of
  * the SocketServer is to manage the lifetime of the AnalysisServer and to
  * encode and decode the JSON messages exchanged with the client.
  */
 class SocketServer {
   final AnalysisServerOptions analysisServerOptions;
 
   /**
    * The function used to create a new SDK using the default SDK.
    */
   final DartSdkManager sdkManager;
 
   final DartSdk defaultSdk;
   final InstrumentationService instrumentationService;
   final DiagnosticServer diagnosticServer;
   final ResolverProvider fileResolverProvider;
   final ResolverProvider packageResolverProvider;
 
   /**
    * The analysis server that was created when a client established a
    * connection, or `null` if no such connection has yet been established.
    */
   AnalysisServer analysisServer;
 
-  /**
-   * The plugins that are defined outside the analysis_server package.
-   */
-  List<Plugin> userDefinedPlugins;
-
   SocketServer(
       this.analysisServerOptions,
       this.sdkManager,
       this.defaultSdk,
       this.instrumentationService,
       this.diagnosticServer,
       this.fileResolverProvider,
       this.packageResolverProvider);
 
   /**
@@ skipping 25 matching lines @@
     analysisServer = new AnalysisServer(
         serverChannel,
         resourceProvider,
         new PubPackageMapProvider(resourceProvider, defaultSdk),
         analysisServerOptions,
         sdkManager,
         instrumentationService,
         diagnosticServer: diagnosticServer,
         fileResolverProvider: fileResolverProvider,
         packageResolverProvider: packageResolverProvider);
-    analysisServer.userDefinedPlugins = userDefinedPlugins;
   }
 }