Skip type checks on native JavaScriptObjects

lib/runtime/_rtti.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.
 
 /* This library defines the association between runtime objects and
  * runtime types.
 */
 
 dart_library.library('dart_runtime/_rtti', null, /* Imports */[
 ], /* Lazy Imports */[
@@ skipping 119 matching lines @@
    */
   function realRuntimeType(obj) {
     let result = checkPrimitiveType(obj);
     if (result !== null) return result;
     // TODO(vsm): Should we treat Dart and JS objects differently here?
     // E.g., we can check if obj instanceof core.Object to differentiate.
     result = obj[_runtimeType];
     if (result) return result;
     result = obj.constructor;
     if (result == Function) {
-      return getFunctionType(obj);
+      // An undecorated Function should have come from
+      // JavaScript.  Treat as untyped.
+      return types.jsobject;
     }
     return result;
   }
   exports.realRuntimeType = realRuntimeType;
 
   function LazyTagged(infoFn) {
     class _Tagged {
       get [_runtimeType]() {return infoFn();}
     }
     return _Tagged;
@@ skipping 20 matching lines @@
     function getter() {
       if (compute == null) return cache;
       cache = compute();
       compute = null;
       return cache;
     }
     tagComputed(value, getter);
   }
   exports.tagMemoized = tagMemoized;
 });