Use null aware operators to clean up code

pkg/analysis_server/lib/plugin/protocol/protocol.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.
 
 /**
  * Support for client code that needs to interact with the requests, responses
  * and notifications that are part of the analysis server's wire protocol.
  */
 library analysis_server.plugin.protocol.protocol;
 
@@ skipping 142 matching lines @@
    */
   final int clientRequestTime;
 
   /**
    * Initialize a newly created [Request] to have the given [id] and [method]
    * name.  If [params] is supplied, it is used as the "params" map for the
    * request.  Otherwise an empty "params" map is allocated.
    */
   Request(this.id, this.method,
       [Map<String, Object> params, this.clientRequestTime])
-      : _params = params != null ? params : new HashMap<String, Object>();
+      : _params = params ?? new HashMap<String, Object>();
 
   /**
    * Return a request parsed from the given json, or `null` if the [data] is
    * not a valid json representation of a request. The [data] is expected to
    * have the following format:
    *
    *   {
    *     'clientRequestTime': millisecondsSinceEpoch
    *     'id': String,
    *     'method': methodName,
@@ skipping 401 matching lines @@
     jsonObject[ID] = id;
     if (error != null) {
       jsonObject[ERROR] = error.toJson();
     }
     if (_result != null) {
       jsonObject[RESULT] = _result;
     }
     return jsonObject;
   }
 }