fix more mixin tests to work in strong mode

pkg/dev_compiler/test/not_yet_strong_tests.dart

 import 'dart:collection';
 import 'package:path/path.dart' as path;
 
 /// 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',
@@ skipping 1067 matching lines @@
   'language/mixin_black_listed_test_01_multi',
   'language/mixin_black_listed_test_02_multi',
   'language/mixin_black_listed_test_03_multi',
   'language/mixin_black_listed_test_04_multi',
   'language/mixin_black_listed_test_05_multi',
   'language/mixin_black_listed_test_06_multi',
   'language/mixin_black_listed_test_07_multi',
   'language/mixin_black_listed_test_08_multi',
   'language/mixin_cyclic_test_01_multi',
   'language/mixin_cyclic_test_02_multi',
-  'language/mixin_extends_field_test',
-  'language/mixin_extends_method_test',
-  'language/mixin_field_test',
   'language/mixin_forwarding_constructor4_test_01_multi',
   'language/mixin_forwarding_constructor4_test_02_multi',
   'language/mixin_forwarding_constructor4_test_03_multi',
   'language/mixin_illegal_constructor_test_01_multi',
   'language/mixin_illegal_constructor_test_02_multi',
   'language/mixin_illegal_constructor_test_03_multi',
   'language/mixin_illegal_constructor_test_04_multi',
   'language/mixin_illegal_constructor_test_05_multi',
   'language/mixin_illegal_constructor_test_06_multi',
   'language/mixin_illegal_constructor_test_07_multi',
@@ skipping 96 matching lines @@
   'language/mixin_invalid_bound_test_08_multi',
   'language/mixin_invalid_bound_test_09_multi',
   'language/mixin_invalid_bound_test_10_multi',
   'language/mixin_invalid_bound_test_none_multi',
   'language/mixin_invalid_inheritance1_test_01_multi',
   'language/mixin_invalid_inheritance1_test_02_multi',
   'language/mixin_invalid_inheritance1_test_03_multi',
   'language/mixin_invalid_inheritance2_test_01_multi',
   'language/mixin_invalid_inheritance2_test_02_multi',
   'language/mixin_invalid_inheritance2_test_03_multi',
-  'language/mixin_lib_extends_field_test',
-  'language/mixin_lib_extends_method_test',
-  'language/mixin_method_test',
   'language/mixin_super_bound2_test_01_multi',
   'language/mixin_super_bound2_test_none_multi',
-  'language/mixin_super_bound_test',
-  'language/mixin_super_constructor2_test',
-  'language/mixin_super_constructor_default_test',
+  'language/mixin_super_bound_test', // tests static errors
   'language/mixin_super_constructor_named_test_01_multi',
-  'language/mixin_super_constructor_named_test_none_multi',
   'language/mixin_super_constructor_positionals_test_01_multi',
-  'language/mixin_super_constructor_positionals_test_none_multi',
-  'language/mixin_super_constructor_test',
   'language/mixin_super_test',
-  'language/mixin_type_parameter4_test',
   'language/mixin_type_parameters_errors_test_01_multi',
   'language/mixin_type_parameters_errors_test_02_multi',
   'language/mixin_type_parameters_errors_test_03_multi',
   'language/mixin_type_parameters_errors_test_04_multi',
   'language/mixin_type_parameters_errors_test_05_multi',
-  'language/mixin_type_parameters_mixin_extends_test',
-  'language/mixin_type_parameters_mixin_test',
-  'language/mixin_type_parameters_simple_test',
-  'language/mixin_type_parameters_super_extends_test',
-  'language/mixin_type_parameters_super_test',
-  'language/mixin_with_two_implicit_constructors_test',
+  'language/mixin_with_two_implicit_constructors_test', // tests static errors
   'language/multiline_newline_test_01_multi',
   'language/multiline_newline_test_02_multi',
   'language/multiline_newline_test_03_multi',
   'language/named_constructor_test_01_multi',
   'language/named_constructor_test_02_multi',
   'language/named_constructor_test_03_multi',
   'language/named_constructor_test_04_multi',
   'language/named_constructor_test_05_multi',
   'language/named_constructor_test_06_multi',
   'language/named_constructor_test_07_multi',
@@ skipping 1189 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)));