Mark durationDisplay as visible-for-testing (#9980)

packages/devtools_app/lib/src/screens/network/network_model.dart

diff --git a/packages/devtools_app/lib/src/screens/network/network_model.dart b/packages/devtools_app/lib/src/screens/network/network_model.dart
index 76af3a9..7154f67 100644
--- a/packages/devtools_app/lib/src/screens/network/network_model.dart
+++ b/packages/devtools_app/lib/src/screens/network/network_model.dart
@@ -4,6 +4,7 @@
 
 import 'package:devtools_shared/devtools_shared.dart';
 import 'package:flutter/material.dart';
+import 'package:meta/meta.dart';
 import 'package:vm_service/vm_service.dart';
 
 import '../../shared/primitives/utils.dart';