fix flutter run in dev/manual_tests (flutter#25817)

Author: goderbauer

dev/manual_tests/lib/main.dart

@@ -4,4 +4,9 @@
 
 import 'package:flutter/widgets.dart';
 
-void main() => runApp(const Center(child: Text('flutter run -t xxx.dart')));
+void main() => runApp(const Directionality(
+  textDirection: TextDirection.ltr,
+  child: Center(
+    child: Text('flutter run -t xxx.dart'),
+  ),
+));