Update l10n.dart

Make the static const

Author: EArminjon

packages/firebase_ui_localizations/lib/src/l10n.dart

@@ -47,8 +47,8 @@ class FirebaseUILocalizations<T extends FirebaseUILocalizationLabels> {
 
   /// Localization delegate that could be provided to the
   /// [MaterialApp.localizationsDelegates].
-  static FirebaseUILocalizationDelegate delegate =
-      const FirebaseUILocalizationDelegate();
+  static const FirebaseUILocalizationDelegate delegate =
+      FirebaseUILocalizationDelegate();
 
   /// Should be used to override labels provided by the library.
   ///