fix #25578, implement @covariant parameter overrides

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Defines the element model. The element model describes the semantic (as
  * opposed to syntactic) structure of Dart code. The syntactic structure of the
  * code is modeled by the [AST structure](../ast/ast.dart).
  *
  * The element model consists of two closely related kinds of objects: elements
@@ skipping 1616 matching lines @@
    * An empty list of parameter elements.
    */
   static const List<ParameterElement> EMPTY_LIST = const <ParameterElement>[];
 
   /**
    * Return the Dart code of the default value, or `null` if no default value.
    */
   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;
 
   /**
    * Return the kind of this parameter.
    */
   ParameterKind get parameterKind;
 
   /**
@@ skipping 374 matching lines @@
   DartType get type;
 
   /**
    * Return a representation of the value of this variable, forcing the value
    * to be computed if it had not previously been computed, or `null` if either
    * this variable was not declared with the 'const' modifier or if the value of
    * this variable could not be computed because of errors.
    */
   DartObject computeConstantValue();
 }