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

pkg/dev_compiler/test/not_yet_strong_tests.dart

 import 'dart:collection';
 
 /// The set of tests that are not yet strong mode safe.
 final notYetStrongTests = new HashSet<String>.from([
   'language/abstract_exact_selector_test_01_multi',
   'language/abstract_factory_constructor_test_00_multi',
   'language/abstract_getter_test_01_multi',
   'language/abstract_runtime_error_test_01_multi',
   'language/abstract_runtime_error_test_02_multi',
   'language/abstract_runtime_error_test_03_multi',
@@ skipping 499 matching lines @@
   'language/constructor_return_test_03_multi',
   'language/constructor_return_test_04_multi',
   'language/constructor_return_test_05_multi',
   'language/constructor_test',
   'language/constructor_with_mixin_test',
   'language/crash_6725_test_01_multi',
   'language/create_unresolved_type_test_01_multi',
   'language/cyclic_class_member_test_01_multi',
   'language/cyclic_constructor_test_01_multi',
   'language/cyclic_default_values_test',
-  'language/cyclic_import_test',
   'language/cyclic_type_variable_test_01_multi',
   'language/cyclic_type_variable_test_02_multi',
   'language/cyclic_type_variable_test_03_multi',
   'language/cyclic_type_variable_test_04_multi',
   'language/cyclic_typedef_test_01_multi',
   'language/cyclic_typedef_test_02_multi',
   'language/cyclic_typedef_test_03_multi',
   'language/cyclic_typedef_test_04_multi',
   'language/cyclic_typedef_test_05_multi',
   'language/cyclic_typedef_test_06_multi',
@@ skipping 1858 matching lines @@
   'corelib/regexp/regexp_test',
   'corelib/regexp/regress-regexp-codeflush_test',
   'corelib/regexp/standalones_test',
   'corelib/string_from_list_test',
   'lib/convert/chunked_conversion_json_encode1_test',
   'lib/convert/chunked_conversion_utf84_test',
   'lib/convert/chunked_conversion_utf88_test',
   'lib/convert/chunked_conversion_utf8_test',
   'lib/convert/line_splitter_test',
   'lib/html/cross_frame_test',
+  'lib/html/custom/constructor_calls_created_synchronously_test',
+  'lib/html/custom/created_callback_test',
+  'lib/html/custom/document_register_basic_test',
+  'lib/html/custom/document_register_type_extensions_test',
+  'lib/html/custom/element_upgrade_test',
   'lib/html/element_test',
   'lib/html/events_test',
   'lib/html/fileapi_test',
   'lib/html/filereader_test',
   'lib/html/fontface_loaded_test',
   'lib/html/js_function_getter_trust_types_test',
   'lib/html/js_typed_interop_side_cast_exp_test',
   'lib/html/js_typed_interop_side_cast_test',
   'lib/html/keyboard_event_test',
   'lib/html/mutationobserver_test',
   'lib/html/postmessage_structured_test',
   'lib/html/resource_http_test',
   'lib/html/transferables_test',
   'lib/html/webgl_1_test',
   'lib/html/wrapping_collections_test',
   // TODO(jmesserly): these are both under "dart:html" as well.
   'js_test',
   'js_util_test'
 ]);