Put inherited readonliness behind a flag,

src/objects.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 2116 matching lines @@
       }
       case MAP_TRANSITION:
       case CONSTANT_TRANSITION:
       case NULL_DESCRIPTOR:
       case ELEMENTS_TRANSITION:
         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[] = { Handle<Object>(name), Handle<Object>(this)};
     return isolate->Throw(*isolate->factory()->NewTypeError(
       "strict_read_only_property", HandleVector(args, ARRAY_SIZE(args))));
   }
   return heap->the_hole_value();
 }
 
 
@@ skipping 11085 matching lines @@
   set_year(Smi::FromInt(year), SKIP_WRITE_BARRIER);
   set_month(Smi::FromInt(month), SKIP_WRITE_BARRIER);
   set_day(Smi::FromInt(day), SKIP_WRITE_BARRIER);
   set_weekday(Smi::FromInt(weekday), SKIP_WRITE_BARRIER);
   set_hour(Smi::FromInt(hour), SKIP_WRITE_BARRIER);
   set_min(Smi::FromInt(min), SKIP_WRITE_BARRIER);
   set_sec(Smi::FromInt(sec), SKIP_WRITE_BARRIER);
 }
 
 } }  // namespace v8::internal