Remove more references to the old plugin system

pkg/analysis_server/lib/src/analysis_server.dart

Index: pkg/analysis_server/lib/src/analysis_server.dart
diff --git a/pkg/analysis_server/lib/src/analysis_server.dart b/pkg/analysis_server/lib/src/analysis_server.dart
index 68804ea6ff23b42d70e4451add2f1e1fc2f6436c..14582d3dc956f080b5dbdc5a99496b546a6a2791 100644
--- a/pkg/analysis_server/lib/src/analysis_server.dart
+++ b/pkg/analysis_server/lib/src/analysis_server.dart
@@ -68,7 +68,6 @@ import 'package:analyzer_plugin/protocol/protocol_common.dart' hide Element;
 import 'package:front_end/src/base/performace_logger.dart';
 import 'package:front_end/src/byte_store/byte_store.dart';
 import 'package:front_end/src/byte_store/file_byte_store.dart';
-import 'package:plugin/plugin.dart';
 import 'package:telemetry/crash_reporting.dart';
 import 'package:telemetry/telemetry.dart' as telemetry;
 import 'package:watcher/watcher.dart';
@@ -253,11 +252,6 @@ class AnalysisServer {
   final ContentCache overlayState = new ContentCache();
 
   /**
-   * The plugins that are defined outside the analysis_server package.
-   */
-  List<Plugin> userDefinedPlugins;
-
-  /**
    * If the "analysis.analyzedFiles" notification is currently being subscribed
    * to (see [generalAnalysisServices]), and at least one such notification has
    * been sent since the subscription was enabled, the set of analyzed files