When to use part/part of versus import/export in Dart?

Question

I do not completely understand the difference between part/part of and import/export when using libraries in Dart. For example:

one.dart:
library one;
part "two.dart";
Class One {
};

and

two.dart:
part of one;
import 'somefile.dart';
Class Two {
}

versus

library one;
import 'two.dart';
Class One {
}

and

library two;
import 'somefile.dart';
export 'somefile.dart';
Class Two {
}

Both scenarios seem to do the same thing. When is it advantageous to use part and part of rather than import? And are there scenarios where import will not work, but part and part of will?

Answer 1

update 2018/03

part and part of is used more and more for code generation scenarios recently (instead of deprecated transformers) and unlikely to go away anytime soon.

Packages like built_value, json_serializable, and many others depend on it.

Discouraged is only the patter where all files of a package are tied together to a single library by having one library file and all other files being part of that library.

original

In Dart, private members are accessible within the same library. With import you import a library and can access only its public members. With part/part of you can split one library into several files and private members are accessible for all code within these files.

see clarifications to below paragraph in above update

Using part / part of is discouraged and the Dart team is considering getting rid of it. I assume they will introduce something like "friend" (https://github.com/dart-lang/sdk/issues/22841), where two libraries can access each other's private members as an alternative before they discontinue part / part of (maybe in a future Dart version).