fix override checking of mixins

pkg/dev_compiler/test/compile_error_tests.dart

 import 'dart:collection';
 import 'package:path/path.dart' as path;
 
 /// The set of tests that are known to fail with compile errors.
 ///
 /// This may be because the test hasn't been made strong mode clean yet, or
 /// because of bugs or missing features in strong mode or analyzer itself.
 final compileErrorTests = new HashSet<String>.from([
   'language/abstract_exact_selector_test_01_multi',
   'language/abstract_factory_constructor_test_00_multi',
@@ skipping 2034 matching lines @@
   'corelib/package_resource_test',
   'corelib/print_test_01_multi',
   'corelib/print_test_none_multi',
   'corelib/splay_tree_test',
   'corelib/string_base_vm_test',
   'corelib/string_from_environment3_test_01_multi',
   'corelib/string_from_environment3_test_02_multi',
   'corelib/string_from_environment3_test_03_multi',
   'corelib/string_from_environment3_test_04_multi',
   'corelib/string_from_environment3_test_05_multi',
-  'corelib/string_replace_test',
   'corelib/symbol_reserved_word_test_04_multi',
   'corelib/symbol_reserved_word_test_05_multi',
   'corelib/symbol_reserved_word_test_07_multi',
   'corelib/symbol_reserved_word_test_08_multi',
   'corelib/symbol_reserved_word_test_10_multi',
   'corelib/symbol_reserved_word_test_11_multi',
   'corelib/symbol_test_01_multi',
   'corelib/symbol_test_02_multi',
   'corelib/symbol_test_03_multi',
   'lib/async/async_await_sync_completer_test',
@@ skipping 336 matching lines @@
   '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'
 ].map((p) => p.replaceAll('/', path.separator)));