Introduce an abstraction to write to a field.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 985472bc9cecc25328e0d99669a124d8bd77fbaa..31ee16c66d210ecb2be186737885ac1a312378b7 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -2610,6 +2610,7 @@ class JSObject: public JSReceiver {
                                        int index);
   inline Object* RawFastPropertyAt(int index);
   inline void FastPropertyAtPut(int index, Object* value);
+  void WriteToField(int descriptor, Object* value);
 
   // Access to in object properties.
   inline int GetInObjectPropertyOffset(int index);
@@ -2900,6 +2901,10 @@ class JSObject: public JSReceiver {
                               ValueType value_type,
                               TransitionFlag flag);
 
+  static void MigrateToNewProperty(Handle<JSObject> object,
+                                   Handle<Map> transition,
+                                   Handle<Object> value);
+
   // Add a property to a slow-case object.
   static void AddSlowProperty(Handle<JSObject> object,
                               Handle<Name> name,