(TBR) improve error message when fails on bots

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:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/domain_context.dart';
 import 'package:analysis_server/src/domain_server.dart';
@@ skipping 78 matching lines @@
       Response.ID: '0',
       Response.ERROR: null
     }));
   }
 
   static String _createContext(AnalysisServer server) {
     ServerDomainHandler handler = new ServerDomainHandler(server);
     Request request = new Request('0', ServerDomainHandler.CREATE_CONTEXT_METHOD);
     request.setParameter(ServerDomainHandler.SDK_DIRECTORY_PARAM, sdkPath);
     Response response = handler.handleRequest(request);
+    if (response.error != null) {
+      fail('Unexpected error: ${response.error.toJson()}');
+    }
     expect(response.error, isNull);
     return response.getResult(ServerDomainHandler.CONTEXT_ID_RESULT);
   }
 }