Skip type checks on native JavaScriptObjects

lib/runtime/dart_runtime.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.
 
 dart_library.library('dart_runtime/dart', null, /* Imports */[
   'dart_runtime/_classes',
   'dart_runtime/_errors',
   'dart_runtime/_generators',
   'dart_runtime/_operations',
   'dart_runtime/_rtti',
@@ skipping 85 matching lines @@
   // Renames
   exports.as = _export(operations.cast);
   exports.is = _export(operations.instanceOf);
 
   // From types
   exportFrom(types, [
     'bottom',
     'definiteFunctionType',
     'dynamic',
     'functionType',
+    'jsobject',
     'typedef',
     'typeName',
     'void',
   ]);
 
   // From rtti
   exportFrom(rtti, [
     'fn',
     'realRuntimeType',
     'runtimeType',
   ]);
 
 });