Add --es-staging flag

src/objects.cc

 // Copyright 2013 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 3056 matching lines @@
         *done = result.IsReadOnly();
         break;
       case INTERCEPTOR: {
         PropertyAttributes attr =
             result.holder()->GetPropertyAttributeWithInterceptor(
                 *object, *name, true);
         *done = !!(attr & READ_ONLY);
         break;
       }
       case CALLBACKS: {
-        if (!FLAG_es5_readonly && result.IsReadOnly()) break;
         *done = true;
         Handle<Object> callback_object(result.GetCallbackObject(), isolate);
         return SetPropertyWithCallback(object, callback_object, name, value,
                                        handle(result.holder()), strict_mode);
       }
       case HANDLER: {
         Handle<JSProxy> proxy(result.proxy());
         return JSProxy::SetPropertyViaPrototypesWithHandler(
             proxy, object, name, value, attributes, strict_mode, done);
       }
       case TRANSITION:
       case NONEXISTENT:
         UNREACHABLE();
         break;
     }
   }
 
   // If we get here with *done true, we have encountered a read-only property.
-  if (!FLAG_es5_readonly) *done = false;
   if (*done) {
     if (strict_mode == kNonStrictMode) return value;
     Handle<Object> args[] = { name, object };
     Handle<Object> error = isolate->factory()->NewTypeError(
         "strict_read_only_property", HandleVector(args, ARRAY_SIZE(args)));
     isolate->Throw(*error);
     return Handle<Object>();
   }
   return isolate->factory()->the_hole_value();
 }
@@ skipping 13375 matching lines @@
 #define ERROR_MESSAGES_TEXTS(C, T) T,
   static const char* error_messages_[] = {
       ERROR_MESSAGES_LIST(ERROR_MESSAGES_TEXTS)
   };
 #undef ERROR_MESSAGES_TEXTS
   return error_messages_[reason];
 }
 
 
 } }  // namespace v8::internal