Use "Object" as the reified type of covariant override parameters.

pkg/analyzer/lib/src/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.
 
 library analyzer.src.dart.element.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 6220 matching lines @@
 
   @override
   void appendTo(StringBuffer buffer) {
     buffer.write(displayName);
     super.appendTo(buffer);
   }
 
   @override
   MethodDeclaration computeNode() =>
       getNodeMatching((node) => node is MethodDeclaration);
+
+  @override
+  FunctionType getReifiedType(DartType objectType) {
+    // Collect the covariant parameters. Do this first so we don't allocate
+    // anything in the common case where there are none.
+    Set<String> covariantNames;
+    for (ParameterElement parameter in parameters) {
+      if (parameter.isCovariant) {
+        covariantNames ??= new Set();
+        covariantNames.add(parameter.name);
+      }
+    }
+
+    if (covariantNames == null) return type;
+
+    List<ParameterElement> covariantParameters = parameters.map((parameter) {
+      if (!covariantNames.contains(parameter.name)) {
+        return parameter;
+      }
+
+      return new ParameterElementImpl.synthetic(
+          parameter.name, objectType, parameter.parameterKind);
+    }).toList();
+
+    return new FunctionElementImpl.synthetic(covariantParameters, returnType)
+        .type;
+  }
 }
 
 /**
  * The constants for all of the modifiers defined by the Dart language and for a
  * few additional flags that are useful.
  *
  * Clients may not extend, implement or mix-in this class.
  */
 class Modifier implements Comparable<Modifier> {
   /**
@@ skipping 2271 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }