Leave "error" key out of response JSON when there is no error.

pkg/analysis_server/test/domain_server_test.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 test.domain.server;
 
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/domain_server.dart';
 import 'package:analysis_server/src/protocol.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 17 matching lines @@
     ServerDomainHandler handler = new ServerDomainHandler(server);
 
     Request createRequest = new Request('0', ServerDomainHandler.CREATE_CONTEXT_METHOD);
     createRequest.setParameter(ServerDomainHandler.SDK_DIRECTORY_PARAM, sdkPath);
     createRequest.setParameter(ServerDomainHandler.CONTEXT_ID_PARAM, 'ctx');
     Response response = handler.handleRequest(createRequest);
     expect(response.id, equals('0'));
     expect(response.error, isNull);
     expect(response.result, isEmpty);
   }
-  
+
   static void createContext_alreadyExists() {
     AnalysisServer server = new AnalysisServer(new MockServerChannel());
     ServerDomainHandler handler = new ServerDomainHandler(server);
 
     Request createRequest = new Request('0', ServerDomainHandler.CREATE_CONTEXT_METHOD);
     createRequest.setParameter(ServerDomainHandler.SDK_DIRECTORY_PARAM, sdkPath);
     createRequest.setParameter(ServerDomainHandler.CONTEXT_ID_PARAM, 'ctx');
     Response response = handler.handleRequest(createRequest);
     expect(response.error, isNull);
     response = handler.handleRequest(createRequest);
@@ skipping 36 matching lines @@
     String contextId = 'ctx';
     Request createRequest = new Request('0', ServerDomainHandler.CREATE_CONTEXT_METHOD);
     createRequest.setParameter(ServerDomainHandler.SDK_DIRECTORY_PARAM, sdkPath);
     createRequest.setParameter(ServerDomainHandler.CONTEXT_ID_PARAM, contextId);
     handler.createContext(createRequest);
 
     Request deleteRequest = new Request('0', ServerDomainHandler.DELETE_CONTEXT_METHOD);
     deleteRequest.setParameter(ServerDomainHandler.CONTEXT_ID_PARAM, contextId);
     Response response = handler.handleRequest(deleteRequest);
     expect(response.toJson(), equals({
-      Response.ID: '0',
-      Response.ERROR: null
+      Response.ID: '0'
     }));
   }
 
   static void shutdown() {
     AnalysisServer server = new AnalysisServer(new MockServerChannel());
     ServerDomainHandler handler = new ServerDomainHandler(server);
 
     expect(server.running, isTrue);
     Request shutdownRequest = new Request('0', ServerDomainHandler.SHUTDOWN_METHOD);
     Response response = handler.handleRequest(shutdownRequest);
     expect(response.toJson(), equals({
-      Response.ID: '0',
-      Response.ERROR: null
+      Response.ID: '0'
     }));
     expect(server.running, isFalse);
   }
 
   static void version() {
     AnalysisServer server = new AnalysisServer(new MockServerChannel());
     ServerDomainHandler handler = new ServerDomainHandler(server);
 
     Request versionRequest = new Request('0', ServerDomainHandler.VERSION_METHOD);
     Response response = handler.handleRequest(versionRequest);
     expect(response.toJson(), equals({
       Response.ID: '0',
-      Response.ERROR: null,
       Response.RESULT: {
         ServerDomainHandler.VERSION_RESULT: '0.0.1'
       }
     }));
   }
 }