fix #27259, implement covariance checking for strong mode and DDC

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 85 matching lines @@
       'compile_time_constant10_test_none_multi': fail,
       'compile_time_constant_a_test': fail,
       'compile_time_constant_b_test': fail,
       'compile_time_constant_d_test': fail,
       'compile_time_constant_k_test_none_multi': fail,
       'compile_time_constant_o_test_none_multi': fail,
       'const_evaluation_test_01_multi': fail,
       'const_switch_test_02_multi': fail,
       'const_switch_test_04_multi': fail,
       'constructor12_test': fail,
-      'covariant_subtyping_tearoff2_test': fail,
-      'covariant_subtyping_tearoff3_test': fail,
-      'covariant_subtyping_unsafe_call2_test': fail,
-      'covariant_subtyping_unsafe_call3_test': fail,
       'custom_await_stack_trace_test': fail,
       'cyclic_type2_test': fail,
       'cyclic_type_test_00_multi': fail,
       'cyclic_type_test_01_multi': fail,
       'cyclic_type_test_02_multi': fail,
       'cyclic_type_test_03_multi': fail,
       'cyclic_type_test_04_multi': fail,
 
       // Deferred libraries are not actually deferred. These tests all test
       // that synchronous access to the library fails.
@@ skipping 788 matching lines @@
 
     for (let action of unittest_tests) {
       try {
         action();
       } catch (e) {
         console.error("Caught error tying to setup test:", e);
       }
     }
   });
 });