(TBR) Fix test failures

pkg/analysis_server/lib/src/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.
 
 library protocol;
 
 import 'dart:convert' show JsonDecoder;
 
 /**
  * Instances of the class [Request] represent a request that was received.
@@ skipping 247 matching lines @@
     result[name] = value;
   }
 
   /**
    * Return a table representing the structure of the Json object that will be
    * sent to the client to represent this response.
    */
   Map<String, Object> toJson() {
     Map jsonObject = new Map();
     jsonObject[ID] = id;
-    jsonObject[ERROR] = error.toJson();
+    if (error == null) {
+      jsonObject[ERROR] = null;
+    } else {
+      jsonObject[ERROR] = error.toJson();
+    }
     if (!result.isEmpty) {
       jsonObject[RESULT] = result;
     }
     return jsonObject;
   }
 }
 
 /**
  * Instances of the class [RequestError] represent information about an error that
  * occurred while attempting to respond to a [Request].
@@ skipping 208 matching lines @@
   /**
    * The response to be returned as a result of the failure.
    */
   final Response response;
 
   /**
    * Initialize a newly created exception to return the given reponse.
    */
   RequestFailure(this.response);
 }