Fix tests to verify defaultArgListString specified as null are null.

pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart

Index: pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart b/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart
index 0ff94078bede7019a50065631bd586079b6c7f93..0a04b695a0be66bff22ac6df98458c4080196749 100644
--- a/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart
+++ b/pkg/analysis_server/lib/src/services/completion/dart/arglist_contributor.dart
@@ -291,8 +291,8 @@ class ArgListContributor extends DartCompletionContributor {
     if (options?.generateFlutterWidgetChildrenBoilerPlate == true) {
       Element element = param.enclosingElement;
       if (element is ConstructorElement) {
-        ClassElement classElement = element.enclosingElement;
-        if (isFlutterWidget(classElement) && param.name == 'children') {
+        if (isFlutterWidget(element.enclosingElement) &&
+            param.name == 'children') {
           return getDefaultStringParameterValue(param);
         }
       }