[xdg_directories] Apply missing test visibility annotation (#194)
Stumbled across the XDG directory library, and noticed something small.
diff --git a/packages/xdg_directories/lib/xdg_directories.dart b/packages/xdg_directories/lib/xdg_directories.dart
index 857b72c..08e8cb6 100644
--- a/packages/xdg_directories/lib/xdg_directories.dart
+++ b/packages/xdg_directories/lib/xdg_directories.dart
@@ -30,6 +30,7 @@
/// replaces the real environment lookups with an override.
///
/// Only available to tests.
+@visibleForTesting
EnvironmentAccessor get xdgEnvironmentOverride => _xdgEnvironmentOverride;
EnvironmentAccessor _xdgEnvironmentOverride;
EnvironmentAccessor _productionGetEnv =
