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

pkg/dev_compiler/test/browser/language_tests.js

Index: pkg/dev_compiler/test/browser/language_tests.js
diff --git a/pkg/dev_compiler/test/browser/language_tests.js b/pkg/dev_compiler/test/browser/language_tests.js
index ad532f125cb30491e40b24e8a4ebcbb5de231e32..529b4fc874ee08da7acf493f8885b2c42616b223 100644
--- a/pkg/dev_compiler/test/browser/language_tests.js
+++ b/pkg/dev_compiler/test/browser/language_tests.js
@@ -103,10 +103,6 @@ define(['dart_sdk', 'async_helper', 'expect', 'unittest', 'is', 'require'],
       '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,
@@ -341,6 +337,7 @@ define(['dart_sdk', 'async_helper', 'expect', 'unittest', 'is', 'require'],
       'stream_transform_test': async_unittest,
       'stream_transformation_broadcast_test': async_unittest,
       'stream_transformer_from_handlers_test': fail,
+      'stream_zones_test': fail, // covariance cast failure
       'timer_cancel1_test': async_unittest,
       'timer_cancel2_test': async_unittest,
       'timer_cancel_test': async_unittest,