MethodElementImpl.getReifiedType() should always create new parameter elements.

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 6643 matching lines @@
     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;
+    // Check whether we have any covariant parameters.
+    // Usually we don't, so we can use the same type.
+    bool hasCovariant = false;
     for (ParameterElement parameter in parameters) {
       if (parameter.isCovariant) {
-        covariantNames ??= new Set();
-        covariantNames.add(parameter.name);
+        hasCovariant = true;
+        break;
       }
     }
 
-    if (covariantNames == null) return type;
+    if (!hasCovariant) {
+      return type;
+    }
 
     List<ParameterElement> covariantParameters = parameters.map((parameter) {
-      if (!covariantNames.contains(parameter.name)) {
-        return parameter;
-      }
-
+      DartType type = parameter.isCovariant ? objectType : parameter.type;
       return new ParameterElementImpl.synthetic(
           parameter.name, objectType, parameter.parameterKind);
     }).toList();
 
     return new FunctionElementImpl.synthetic(covariantParameters, returnType)
         .type;
   }
 }
 
 /**
@@ skipping 2409 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }