fix #29146, downgrade import error to warning 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 811 matching lines @@
   'language/getter_override_test_01_multi',
   'language/getter_override_test_02_multi',
   'language/getter_override_test_03_multi',
   'language/getter_parameters_test_01_multi',
   'language/getter_parameters_test_02_multi',
   'language/getter_parameters_test_03_multi',
   'language/getter_parameters_test_04_multi',
   'language/getter_setter_in_lib_test',
   'language/getters_setters2_test_02_multi',
   'language/getters_setters2_test_03_multi',
-  'language/hidden_import_test_01_multi',
-  'language/hidden_import_test_02_multi',
   'language/identical_const_test_01_multi',
   'language/identical_const_test_02_multi',
   'language/identical_const_test_03_multi',
   'language/identical_const_test_04_multi',
   'language/if_null_assignment_behavior_test_03_multi',
   'language/if_null_assignment_behavior_test_04_multi',
   'language/if_null_assignment_behavior_test_13_multi',
   'language/if_null_assignment_behavior_test_14_multi',
   'language/if_null_assignment_behavior_test_15_multi',
   'language/if_null_assignment_behavior_test_29_multi',
@@ skipping 1570 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)));