gistfile1.txt

v8::Object::Set(obj, key, value)
 v8::Object* => v8::internal::JSObject** (MAKE_OPEN_HANDLE api.h, this is the external -> internal mapping, that same file also seems to have a macro for internal -> external mapping)
 v8::internal::SetProperty(i::Object obj, i::Object key, i::Object val, flags)
   implemented handles.cc:268
   CALL_HEAP_FUNCTION(isolate, i::Object::SetProperty())
    => JSObject inherits from JSReceiver
       JSReceiver::SetProperty objects.cc:1964 -> objects.cc:2619
        => JSObject::SetPropertyForResult
         => SetNormalizedProperty ( for normal, non-global, etc)
 	  => JSObject::properties_dictionary() => JSObject::properties()
            defined in objects_inl.h:1183 by macro ACCESSORS


These are the macros that actually access the data:
ACCESSORS(JSObject, properties, FixedArray, kPropertiesOffset)
#define ACCESSORS(holder, name, type, offset)                           \
  type* holder::name() { return type::cast(READ_FIELD(this, offset)); } \
  void holder::set_##name(type* value, WriteBarrierMode mode) {         \
    WRITE_FIELD(this, offset, value);                                   \
    CONDITIONAL_WRITE_BARRIER(GetHeap(), this, offset, value, mode);    \
  }

#define READ_FIELD(p, offset) \
  (*reinterpret_cast<Object**>(FIELD_ADDR(p, offset)))

#define FIELD_ADDR(p, offset) \
  (reinterpret_cast<byte*>(p) + offset - kHeapObjectTag)