createEntity method
Implementation
ModelEntity createEntity(String name, [int uid = 0]) {
final id = lastEntityId.id + 1;
if (uid != 0 && containsUid(uid)) {
throw StateError('uid already exists: $uid');
}
final uniqueUid = uid == 0 ? generateUid() : uid;
var entity = ModelEntity.create(IdUid(id, uniqueUid), name, this);
entities.add(entity);
lastEntityId = entity.id;
return entity;
}