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

pkg/analysis_server/test/domain_context_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.context;
 
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/source_io.dart';
 import 'package:analysis_server/src/analysis_server.dart';
@@ skipping 28 matching lines @@
     request.setParameter(ContextDomainHandler.SOURCES_PARAM, []);
     request.setParameter(ContextDomainHandler.CHANGES_PARAM, {
       ContextDomainHandler.ADDED_PARAM : ['ffile:/one.dart'],
       ContextDomainHandler.MODIFIED_PARAM : ['ffile:/two.dart'],
       ContextDomainHandler.REMOVED_PARAM : ['ffile:/three.dart']
     });
     expect(server.contextWorkQueue, isEmpty);
     Response response = handler.handleRequest(request);
     expect(server.contextWorkQueue, hasLength(1));
     expect(response.toJson(), equals({
-      Response.ID: '0',
-      Response.ERROR: null
+      Response.ID: '0'
     }));
   }
 
   static void createChangeSet() {
     AnalysisServer server = new AnalysisServer(new MockServerChannel());
     Request request = new Request('0', ContextDomainHandler.APPLY_CHANGES_NAME);
     ContextDomainHandler handler = new ContextDomainHandler(server);
     SourceFactory sourceFactory = new SourceFactory([new FileUriResolver()]);
     ChangeSet changeSet = handler.createChangeSet(request, sourceFactory,
         new RequestDatum(request, ContextDomainHandler.CHANGES_PARAM, {
@@ skipping 11 matching lines @@
     AnalysisServer server = new AnalysisServer(new MockServerChannel());
     String contextId = _createContext(server);
     Map<String, Object> options = new Map<String, Object>();
     ContextDomainHandler handler = new ContextDomainHandler(server);
 
     Request request = new Request('0', ContextDomainHandler.SET_OPTIONS_NAME);
     request.setParameter(ContextDomainHandler.CONTEXT_ID_PARAM, contextId);
     request.setParameter(ContextDomainHandler.OPTIONS_PARAM, options);
     Response response = handler.handleRequest(request);
     expect(response.toJson(), equals({
-      Response.ID: '0',
-      Response.ERROR: null
+      Response.ID: '0'
     }));
   }
 
   static void setPrioritySources_empty() {
     AnalysisServer server = new AnalysisServer(new MockServerChannel());
     String contextId = _createContext(server);
     List<String> sources = new List<String>();
     ContextDomainHandler handler = new ContextDomainHandler(server);
 
     Request request = new Request('0', ContextDomainHandler.SET_PRIORITY_SOURCES_NAME);
     request.setParameter(ContextDomainHandler.CONTEXT_ID_PARAM, contextId);
     request.setParameter(ContextDomainHandler.SOURCES_PARAM, sources);
     Response response = handler.handleRequest(request);
     expect(response.toJson(), equals({
-      Response.ID: '0',
-      Response.ERROR: null
+      Response.ID: '0'
     }));
   }
 
   static void setPrioritySources_nonEmpty() {
     AnalysisServer server = new AnalysisServer(new MockServerChannel());
     String contextId = _createContext(server);
     List<String> sources = new List<String>();
     sources.add("foo.dart");
     ContextDomainHandler handler = new ContextDomainHandler(server);
 
     Request request = new Request('0', ContextDomainHandler.SET_PRIORITY_SOURCES_NAME);
     request.setParameter(ContextDomainHandler.CONTEXT_ID_PARAM, contextId);
     request.setParameter(ContextDomainHandler.SOURCES_PARAM, sources);
     Response response = handler.handleRequest(request);
     expect(response.toJson(), equals({
-      Response.ID: '0',
-      Response.ERROR: null
+      Response.ID: '0'
     }));
   }
 
   static String _createContext(AnalysisServer server) {
     String contextId = "context${contextIdCounter++}";
     ServerDomainHandler handler = new ServerDomainHandler(server);
     Request request = new Request('0', ServerDomainHandler.CREATE_CONTEXT_METHOD);
     request.setParameter(ServerDomainHandler.SDK_DIRECTORY_PARAM, sdkPath);
     request.setParameter(ServerDomainHandler.CONTEXT_ID_PARAM, contextId);
     Response response = handler.handleRequest(request);
     if (response.error != null) {
       fail('Unexpected error: ${response.error.toJson()}');
     }
     return contextId;
   }
 }