[1.5.4] Fix accessibility level

Runtime/cupertino/app.cs

             List<Locale> supportedLocales = null,
             bool showPerformanceOverlay = false
         ) : base(key: key) {
-            D.assert(routes != null);
-            D.assert(navigatorObservers != null);
-            D.assert(title != null);
-            D.assert(showPerformanceOverlay != null);
+            // D.assert(routes != null);
+            // D.assert(navigatorObservers != null);
+            // D.assert(title != null);
+            // D.assert(showPerformanceOverlay != null);
 
             supportedLocales = supportedLocales ?? new List<Locale> {new Locale("en", "US")};
             this.navigatorKey = navigatorKey;

Runtime/cupertino/theme.cs

         public static readonly Color _kDefaultBarDarkBackgroundColor = new Color(0xB7212121);
     }
 
-    class CupertinoTheme : StatelessWidget {
+    public class CupertinoTheme : StatelessWidget {
         public CupertinoTheme(
             CupertinoThemeData data,
             Widget child,