fix #25578, implement @covariant parameter overrides

pkg/analyzer/test/src/context/mock_sdk.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.test.src.context.mock_sdk;
 
 import 'package:analyzer/dart/element/element.dart' show LibraryElement;
 import 'package:analyzer/file_system/file_system.dart' as resource;
 import 'package:analyzer/file_system/memory_file_system.dart' as resource;
 import 'package:analyzer/src/context/cache.dart';
@@ skipping 230 matching lines @@
 class Map<K, V> extends Object {
   Iterable<K> get keys => null;
   V operator [](K key) => null;
   void operator []=(K key, V value) {}
 }
 
 external bool identical(Object a, Object b);
 
 void print(Object object) {}
 
-const proxy = const _Proxy();
 class _Proxy { const _Proxy(); }
+const Object proxy = const _Proxy();
 
-class _Override {
-  const _Override();
-}
+class _Override { const _Override(); }
 const Object override = const _Override();
 ''');
 
 const _MockSdkLibrary _LIB_FOREIGN_HELPER = const _MockSdkLibrary(
     'dart:_foreign_helper',
     '$sdkRoot/lib/_foreign_helper/_foreign_helper.dart',
     '''
 library dart._foreign_helper;
 
 JS(String typeDescription, String codeTemplate,
@@ skipping 232 matching lines @@
 
   @override
   AnalysisCache createCacheFromSourceFactory(SourceFactory factory) {
     if (factory == null) {
       return super.createCacheFromSourceFactory(factory);
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }