Remove special list behavior in analysis server protocol.

pkg/analysis_server/test/search/type_hierarchy_test.dart

Index: pkg/analysis_server/test/search/type_hierarchy_test.dart
diff --git a/pkg/analysis_server/test/search/type_hierarchy_test.dart b/pkg/analysis_server/test/search/type_hierarchy_test.dart
index ab425529ad07076a3634f62804d3bad6027739e1..74152b49efa163dba3dc013bf0cf79d2545f0229 100644
--- a/pkg/analysis_server/test/search/type_hierarchy_test.dart
+++ b/pkg/analysis_server/test/search/type_hierarchy_test.dart
@@ -45,7 +45,7 @@ main() {
 ''');
     return waitForTasksFinished().then((_) {
       return _getTypeHierarchy('main() {').then((items) {
-        expect(items, isEmpty);
+        expect(items, isNull);
       });
     });
   }