fix #25578, implement @covariant parameter overrides

pkg/analyzer/test/src/context/mock_sdk.dart

Index: pkg/analyzer/test/src/context/mock_sdk.dart
diff --git a/pkg/analyzer/test/src/context/mock_sdk.dart b/pkg/analyzer/test/src/context/mock_sdk.dart
index 91cc63584c2cd3c13dd4f059359a422324901b98..e0a9575c1f5e82d173d6a2768cbdce71fb0e1443 100644
--- a/pkg/analyzer/test/src/context/mock_sdk.dart
+++ b/pkg/analyzer/test/src/context/mock_sdk.dart
@@ -248,12 +248,10 @@ external bool identical(Object a, Object b);
 
 void print(Object object) {}
 
-const proxy = const _Proxy();
 class _Proxy { const _Proxy(); }
+const Object proxy = const _Proxy();
 
-class _Override {
-  const _Override();
-}
+class _Override { const _Override(); }
 const Object override = const _Override();
 ''');