fix #25578, implement @covariant parameter overrides

pkg/analyzer/lib/src/generated/type_system.dart

Index: pkg/analyzer/lib/src/generated/type_system.dart
diff --git a/pkg/analyzer/lib/src/generated/type_system.dart b/pkg/analyzer/lib/src/generated/type_system.dart
index b19ae14f90edc13ed08740c472c8980f39d4def1..6e61466a223756ea6523840552453872838023f6 100644
--- a/pkg/analyzer/lib/src/generated/type_system.dart
+++ b/pkg/analyzer/lib/src/generated/type_system.dart
@@ -673,18 +673,30 @@ class StrongTypeSystemImpl extends TypeSystem {
         provider.dynamicType;
   }
 
-  /**
-   * Check that [f1] is a subtype of [f2].
-   *
-   * This will always assume function types use fuzzy arrows, in other words
-   * that dynamic parameters of f1 and f2 are treated as bottom.
-   */
+  /// Check that [f1] is a subtype of [f2].
+  ///
+  /// This will always assume function types use fuzzy arrows, in other words
+  /// that dynamic parameters of f1 and f2 are treated as bottom.
   bool _isFunctionSubtypeOf(FunctionType f1, FunctionType f2) {
     return FunctionTypeImpl.relate(
         f1,
         f2,
-        (DartType t1, DartType t2) =>
-            _isSubtypeOf(t2, t1, null, dynamicIsBottom: true),
+        (t1, t2, _, __) => _isSubtypeOf(t2, t1, null, dynamicIsBottom: true),
+        instantiateToBounds,
+        returnRelation: isSubtypeOf);
+  }
+
+  /// Check that [f1] is a subtype of [f2] for an override.
+  ///
+  /// This is different from the normal function subtyping in two ways:
+  /// - we know the function types are strict arrows,
+  /// - it allows opt-in covariant parameters.
+  bool isOverrideSubtypeOf(FunctionType f1, FunctionType f2) {
+    return FunctionTypeImpl.relate(
+        f1,
+        f2,
+        (t1, t2, t1Covariant, _) =>
+            isSubtypeOf(t2, t1) || t1Covariant && isSubtypeOf(t1, t2),
         instantiateToBounds,
         returnRelation: isSubtypeOf);
   }