Mark test strong clean for 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 1582 matching lines @@
   'language/super_operator_index8_test',
   'language/super_operator_index_test_01_multi',
   'language/super_operator_index_test_02_multi',
   'language/super_operator_index_test_03_multi',
   'language/super_operator_index_test_04_multi',
   'language/super_operator_index_test_05_multi',
   'language/super_operator_index_test_06_multi',
   'language/super_operator_index_test_07_multi',
   'language/super_operator_test',
   'language/super_test',
-  'language/switch6_test',
   'language/switch_bad_case_test_01_multi',
   'language/switch_bad_case_test_02_multi',
   'language/switch_case_test_00_multi',
   'language/switch_case_test_01_multi',
   'language/switch_case_test_02_multi',
   'language/switch_case_test_none_multi',
   'language/switch_fallthru_test',
   'language/symbol_conflict_test',
   'language/symbol_literal_test_01_multi',
   'language/sync_generator1_test_01_multi',
@@ skipping 825 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'
 ]);