function extendNotExistingKeys(target, extension) {
  (0, _object.objectEach)(extension, function (value, key) {
    if ((0, _mixed.isUndefined)(target[key])) {
      target[key] = value;
    }
  });
  return target;
}
Perform shallow extend of a target object with only this extension's properties which doesn't exist in the target.

@param {Object} target An object that will receive the new properties.
@param {Object} extension An object containing additional properties to merge into the target.