fix errors and warnings in DDC

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 1468 matching lines @@
   'language/regress_19413_test_01_multi',
   'language/regress_19728_test',
   'language/regress_20394_test_01_multi',
   'language/regress_21793_test_01_multi',
   'language/regress_21912_test_01_multi',
   'language/regress_21912_test_02_multi',
   'language/regress_22438_test',
   'language/regress_22936_test_01_multi',
   'language/regress_23038_test_01_multi',
   'language/regress_23051_test_01_multi',
-  'language/regress_23089_test', // issue 28450
   'language/regress_23408_test',
   'language/return_type_test',
   'language/rewrite_implicit_this_test_01_multi',
   'language/scope_variable_test_01_multi',
   'language/setter_no_getter_call_test_01_multi',
   'language/setter_override_test_00_multi',
   'language/setter_override_test_01_multi',
   'language/setter_override_test_02_multi',
   'language/setter_override_test_03_multi',
   'language/static_field1_test',
@@ skipping 918 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'
 ]);