Correct target of ADD_MISSING_REQUIRED_ARGUMENT fixes (flutter-intellij#788)

pkg/analysis_server/lib/src/services/correction/fix_internal.dart

Index: pkg/analysis_server/lib/src/services/correction/fix_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
index 9c19900a1d90f438e6406a8226d635e2ce24c6e2..34bd735d4b6a7a18ef51dee8bde2cda43cebcf6c 100644
--- a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
@@ -598,7 +598,7 @@ class FixProcessor {
       // In the future consider better values than null for specific element types.
       sb.append('$paramName: null');
 
-      _insertBuilder(sb, targetElement);
+      _insertBuilder(sb, null);
       _addFix(DartFixKind.ADD_MISSING_REQUIRED_ARGUMENT, [paramName]);
     }
   }