function cloneMapObject(originalObject, objectId) {
  const object    = copyObject(originalObject)
  const conflicts = copyObject(originalObject ? originalObject[CONFLICTS] : undefined)
  Object.defineProperty(object, OBJECT_ID, {value: objectId})
  Object.defineProperty(object, CONFLICTS, {value: conflicts})
  return object
}
Creates a writable copy of an immutable map object. If `originalObject`
is undefined, creates an empty object with ID `objectId`.