fix #25578, implement @covariant parameter overrides

pkg/analyzer/lib/dart/element/element.dart

Index: pkg/analyzer/lib/dart/element/element.dart
diff --git a/pkg/analyzer/lib/dart/element/element.dart b/pkg/analyzer/lib/dart/element/element.dart
index b829beed070b230382d275e103e98208b085ea62..d6c087f2808dffd10bced469fb68b40136b255c4 100644
--- a/pkg/analyzer/lib/dart/element/element.dart
+++ b/pkg/analyzer/lib/dart/element/element.dart
@@ -1634,6 +1634,12 @@ abstract class ParameterElement
   String get defaultValueCode;
 
   /**
+   * Return `true` if this parameter is covariant, meaning it is allowed to have
+   * a narrower type in an override.
+   */
+  bool get isCovariant;
+
+  /**
    * Return `true` if this parameter is an initializing formal parameter.
    */
   bool get isInitializingFormal;