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

pkg/dev_compiler/test/browser/language_tests.js

 // Copyright (c) 2015, 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.
 
 define(['dart_sdk', 'async_helper', 'expect', 'unittest', 'is', 'require'],
       function(dart_sdk, async_helper, expect, unittest, is, require) {
   'use strict';
 
   async_helper = async_helper.async_helper;
   let minitest = expect.minitest;
@@ skipping 254 matching lines @@
 
       // https://github.com/dart-lang/sdk/issues/26124
       'prefix10_negative_test': fail,
 
       'library_prefixes_test1': 'helper',
       'library_prefixes_test2': 'helper',
       'top_level_prefixed_library_test': 'helper',
 
     },
 
+    'language/covariant_override': {},
+
     'corelib': {
       'apply2_test': fail,
       'apply3_test': fail,
       'apply_test': fail,
       'big_integer_parsed_arith_vm_test': fail,
       'big_integer_parsed_div_rem_vm_test': fail,
       'big_integer_parsed_mul_div_vm_test': fail,
       'bit_twiddling_bigint_test': fail,
       'collection_length_test': skip_timeout,
       'compare_to2_test': fail,
@@ skipping 502 matching lines @@
         window.onerror = function(message, url, line, column, error) {
           console.warn('Asynchronous error in ' + name + ': ' + message);
           if (!error) {
             error = new Error(message);
           }
           finish(error);
         };
 
         async_helper.asyncTestInitialize(finish);
         if (has('slow')) this.timeout(10000);
- 
+
         var result;
         try {
           var result = mainLibrary.main();
           if (result && !(result instanceof dart_sdk.async.Future)) {
             result = null;
           }
         } catch (e) {
           finish(e);
         }
 
@@ skipping 68 matching lines @@
 
     for (let action of unittest_tests) {
       try {
         action();
       } catch (e) {
         console.error("Caught error tying to setup test:", e);
       }
     }
   });
 });