write method
writes toWrite to storage according to LocalValue.basePath and LocalValue.documentType.
It is possible to put a path in id (i.e. write('my/path/${user.id}')),
but is not recommended because subsequent lookups will need to be pathed the same. instead, put
path segments when constructing this LocalValue.
Implementation
Future<void> write(String id, T toWrite) {
return super.writeValue(id, toWrite);
}