Change analysis server protocol to omit empty lists when optional.

pkg/analysis_server/test/domain_server_test.dart

Index: pkg/analysis_server/test/domain_server_test.dart
diff --git a/pkg/analysis_server/test/domain_server_test.dart b/pkg/analysis_server/test/domain_server_test.dart
index ab0003295ae82d58eb5a41c37c2879dcf63de487..a383691e58353024374a3e7f7ded184ff722ed80 100644
--- a/pkg/analysis_server/test/domain_server_test.dart
+++ b/pkg/analysis_server/test/domain_server_test.dart
@@ -53,7 +53,7 @@ main() {
         expect(server.serverServices, isEmpty);
         // send request
         Request request = new ServerSetSubscriptionsParams(
-            [ServerService.STATUS]).toRequest('0');
+            subscriptions: [ServerService.STATUS]).toRequest('0');
         var response = handler.handleRequest(request);
         expect(response, isResponseSuccess('0'));
         // set of services has been changed