Better is/as check handling. With this CL, the subtype function in

test/browser/language_tests.js

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 (function() {
   'use strict';
 
   let dart_sdk = dart_library.import('dart_sdk');
   dart_sdk._isolate_helper.startRootIsolate(function() {}, []);
   let async_helper = dart_library.import('async_helper').async_helper;
@@ skipping 407 matching lines @@
       'int_modulo_arith_test_none_multi': fail,
       'integer_to_radix_string_test': fail,
       'integer_to_string_test_01_multi': fail,
       'iterable_empty_test': fail,
       'iterable_expand_test': fail,
       'iterable_generate_test': fail,
       'iterable_return_type_test_01_multi': fail,
       'iterable_return_type_test_02_multi': fail,
       'iterable_return_type_test_none_multi': fail,
       'json_map_test': fail,
-      'linked_hash_map_from_iterables_test': fail,
       'list_fill_range_test': fail,
-      'list_filled_type_argument_test': fail,
       'list_get_range_test': fail,
       'list_replace_range_test': fail,
       'list_set_all_test': fail,
       'list_to_string2_test': fail,
       'list_to_string_test': fail,
       'main_test': fail,
-      'map_from_iterable_test': fail,
-      'map_from_iterables_test': fail,
       'map_keys2_test': fail,
       'map_to_string_test': fail,
-      'map_values2_test': fail,
-      'map_values3_test': fail,
-      'map_values4_test': fail,
       'nan_infinity_test_01_multi': fail,
       'null_nosuchmethod_test': fail,
       'null_test': fail,
       'num_sign_test': fail,
       'queue_test': fail,
       'regress_r21715_test': fail,
-      'splay_tree_from_iterable_test': fail,
-      'splay_tree_from_iterables_test': fail,
       'throw_half_surrogate_pair_test_02_multi': fail,
       'stacktrace_current_test': fail,
       'stacktrace_fromstring_test': fail,
       'string_from_environment2_test': fail,
       'string_from_environment_test': fail,
       'string_fromcharcodes_test': fail,
       'string_operations_with_null_test': fail,
       'symbol_reserved_word_test_06_multi': fail,
       'symbol_reserved_word_test_09_multi': fail,
       'symbol_reserved_word_test_12_multi': fail,
@@ skipping 82 matching lines @@
 
         if (!async_helper.asyncTestStarted) done();
       });
     }
   }
 
   for (let action of unittest_tests) {
     action();
   }
 })();