Unify the server and plugin versions of the generators

pkg/analyzer_plugin/lib/protocol/protocol.dart

 // Copyright (c) 2017, 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:analyzer_plugin/protocol/protocol_generated.dart';
 import 'package:analyzer_plugin/src/protocol/protocol_internal.dart';
 
 /**
  * An interface for enumerated types in the protocol.
  *
@@ skipping 26 matching lines @@
 
   /**
    * The name of the event that triggered the notification.
    */
   final String event;
 
   /**
    * A table mapping the names of notification parameters to their values, or
    * `null` if there are no notification parameters.
    */
-  Map<String, Object> params;
+  final Map<String, Object> params;
 
   /**
    * Initialize a newly created [Notification] to have the given [event] name.
    * If [params] is provided, it will be used as the params; otherwise no
    * params will be used.
    */
   Notification(this.event, [this.params]);
 
   /**
    * Initialize a newly created instance based on the given JSON data.
@@ skipping 514 matching lines @@
     jsonObject[ID] = id;
     if (error != null) {
       jsonObject[ERROR] = error.toJson();
     }
     if (result != null) {
       jsonObject[RESULT] = result;
     }
     return jsonObject;
   }
 }