Fix bug 344: always keep attributes of existing properties.

src/objects.cc

Index: src/objects.cc
===================================================================
--- src/objects.cc	(revision 1905)
+++ src/objects.cc	(working copy)
@@ -1736,8 +1736,10 @@
       }
       return ConvertDescriptorToField(name, value, attributes);
     case CONSTANT_FUNCTION:
+      // Only replace the function if necessary.
       if (value == result->GetConstantFunction()) return value;
-      // Only replace the function if necessary.
+      // Preserve the attributes of this existing property.
+      attributes = result->GetAttributes();
       return ConvertDescriptorToFieldAndMapTransition(name, value, attributes);
     case CALLBACKS:
       return SetPropertyWithCallback(result->GetCallbackObject(),
@@ -1817,8 +1819,10 @@
       }
       return ConvertDescriptorToField(name, value, attributes);
     case CONSTANT_FUNCTION:
+      // Only replace the function if necessary.
       if (value == result->GetConstantFunction()) return value;
-      // Only replace the function if necessary.
+      // Preserve the attributes of this existing property.
+      attributes = result->GetAttributes();
       return ConvertDescriptorToFieldAndMapTransition(name, value, attributes);
     case CALLBACKS:
     case INTERCEPTOR: