According to ES-262, Object.seal and Object.freeze should return object's its...

src/v8natives.js

Index: src/v8natives.js
===================================================================
--- src/v8natives.js	(revision 5169)
+++ src/v8natives.js	(working copy)
@@ -749,7 +749,7 @@
     if (desc.isConfigurable()) desc.setConfigurable(false);
     DefineOwnProperty(obj, name, desc, true);
   }  
-  ObjectPreventExtension(obj);
+  return ObjectPreventExtension(obj);
 }
 
 
@@ -766,7 +766,7 @@
     if (desc.isConfigurable()) desc.setConfigurable(false);
     DefineOwnProperty(obj, name, desc, true);
   }  
-  ObjectPreventExtension(obj);
+  return ObjectPreventExtension(obj);
 }