Support merging with distinct attributes.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 12318ba3819357601f919f9c7fcef41d220fcaef..b3c8d55cd81366a953d41a0f488ca2a888671476 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -7291,7 +7291,6 @@ MaybeObject* DescriptorArray::Merge(int verbatim,
     Name* key = GetKey(descriptor);
     PropertyDetails details = GetDetails(descriptor);
     PropertyDetails other_details = other->GetDetails(descriptor);
-    ASSERT(details.attributes() == other_details.attributes());
 
     if (details.type() == FIELD || other_details.type() == FIELD ||
         (details.type() == CONSTANT_FUNCTION &&
@@ -7301,7 +7300,7 @@ MaybeObject* DescriptorArray::Merge(int verbatim,
           details.representation().generalize(other_details.representation());
       FieldDescriptor d(key,
                         current_offset++,
-                        details.attributes(),
+                        other_details.attributes(),
                         representation);
       result->Set(descriptor, &d, witness);
     } else {