Adjust test/material/popup_menu_test.dart for Dart 2 semantics. (#14644)

packages/flutter/test/material/popup_menu_test.dart

@@ -215,7 +215,12 @@ void main() {
         child: const Text('XXX'),
       ),
     );
-    final WidgetPredicate popupMenu = (Widget widget) => widget.runtimeType.toString() == '_PopupMenu';
+    final WidgetPredicate popupMenu = (Widget widget) {
+      final String widgetType = widget.runtimeType.toString();
+      // TODO(mraleph): Remove the old case below.
+      return widgetType == '_PopupMenu<int>' // normal case
+          || widgetType == '_PopupMenu'; // for old versions of Dart that don't reify method type arguments
+    };
 
     Future<Null> openMenu(TextDirection textDirection, Alignment alignment) async {
       return TestAsyncUtils.guard(() async {