(TBR) improve error message when fails on bots

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 850a94e2d06cdaeaf42b20ba4c2369c9031763ff..953f8eb64764855f4bb3999d6282c748fa0dc192 100644
--- a/pkg/analysis_server/test/domain_context_test.dart
+++ b/pkg/analysis_server/test/domain_context_test.dart
@@ -96,6 +96,9 @@ class ContextDomainHandlerTest {
     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);
   }