Remove one more case behind --es5_readonly flag.

src/hydrogen.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4666 matching lines @@
       // If the inherited property could induce readonly-ness, bail out.
       if (proto_result.IsReadOnly() || !proto_result.IsCacheable()) {
         Bailout("improper object on prototype chain for store");
         return NULL;
       }
       // We only need to check up to the preexisting property.
       proto = proto_result.holder();
     } else {
       // Otherwise, find the top prototype.
       while (proto->GetPrototype()->IsJSObject()) proto = proto->GetPrototype();
+      ASSERT(proto->GetPrototype()->IsNull());
     }
     ASSERT(proto->IsJSObject());
     AddInstruction(new(zone()) HCheckPrototypeMaps(
         Handle<JSObject>(JSObject::cast(type->prototype())),
         Handle<JSObject>(JSObject::cast(proto))));
   }
 
   int index = ComputeLoadStoreFieldIndex(type, name, lookup);
   bool is_in_object = index < 0;
   int offset = index * kPointerSize;
@@ skipping 4429 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify(false);  // No full verify.
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal