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

pkg/analysis_server/test/domain_context_test.dart

Index: pkg/analysis_server/test/domain_context_test.dart
diff --git a/pkg/analysis_server/test/domain_context_test.dart b/pkg/analysis_server/test/domain_context_test.dart
index 2181565ff7216d509f0d316f4da0636329425c42..b9f84ef79567fb6f288d10d63bbb640e5efe3653 100644
--- a/pkg/analysis_server/test/domain_context_test.dart
+++ b/pkg/analysis_server/test/domain_context_test.dart
@@ -46,8 +46,7 @@ class ContextDomainHandlerTest {
     Response response = handler.handleRequest(request);
     expect(server.contextWorkQueue, hasLength(1));
     expect(response.toJson(), equals({
-      Response.ID: '0',
-      Response.ERROR: null
+      Response.ID: '0'
     }));
   }
 
@@ -79,8 +78,7 @@ class ContextDomainHandlerTest {
     request.setParameter(ContextDomainHandler.OPTIONS_PARAM, options);
     Response response = handler.handleRequest(request);
     expect(response.toJson(), equals({
-      Response.ID: '0',
-      Response.ERROR: null
+      Response.ID: '0'
     }));
   }
 
@@ -95,8 +93,7 @@ class ContextDomainHandlerTest {
     request.setParameter(ContextDomainHandler.SOURCES_PARAM, sources);
     Response response = handler.handleRequest(request);
     expect(response.toJson(), equals({
-      Response.ID: '0',
-      Response.ERROR: null
+      Response.ID: '0'
     }));
   }
 
@@ -112,8 +109,7 @@ class ContextDomainHandlerTest {
     request.setParameter(ContextDomainHandler.SOURCES_PARAM, sources);
     Response response = handler.handleRequest(request);
     expect(response.toJson(), equals({
-      Response.ID: '0',
-      Response.ERROR: null
+      Response.ID: '0'
     }));
   }