Conditional import broken in Dart 2

[daniel-v]

Dart VM version: 2.0.0 (Fri Aug 3 10:53:23 2018 +0200) on "windows_x64"

Conditional import/export worked in last version of Dart 1 and doesn't seem to work any longer.

export 'feature/myinterface.dart' if (somevar) 'impls/someimpl.dart';

Invoking it

$> dart -Dsomevar=true bin\main.dart

Interestingly enough, I see it working with built-in libraries. dart.library.io evals while custom env values do not.

This prevents me from upgrading to Dart 2 in some of my projects.

[zoechi]

I think you need to pass --conditional_directives

https://github.com/dart-lang/sdk/blob/master/CHANGELOG.md#experimental-language-features shows it with - instead of _, but dart -v -h shows it with _

[lrhn]

The version of conditional imports that made it into Dart 2 only allows the dart.library.x directives.
Allowing arbitrary directives interferes with staged modular compilation (the front-end compiler doesn't necessarily know what variables will be set to what). It means that you can only really use the conditional import/export feature for selecting code based on the backend (VM or browser) that the code is being compiled for.

[zoechi]

@lrhn I thought remembering such a discussion but couldn't find anything in the changelog.
Do you think it might be worth mentioning?
When the experimental feature was added it was mentioned https://github.com/dart-lang/sdk/blob/master/CHANGELOG.md#1150---2016-03-09

[daniel-v]

So if I get this right, arbitrary directives are no longer supported.

Are there plans to readd support for it or should I look for another solution?

[matanlurey]

I don't believe there are plans to support arbitrary directives @daniel-v.

(@zoechi: This feature was experimental in Dart 1.x, hence the lack of documentation or support, so it really shouldn't have been relied on and mentioning how it changed was probably not a high priority)

[natebosch]

Yeah this was discussed on #32960

tl;dr; config specific imports are only supported on dart.library.* conditions. I'm not sure if we have an issue tracking the language specification changes for this restriction.

[matanlurey]

Closing as WAI.

See #33347 (comment) and #32960.

[lrhn]

That specification just says that implementations may restrict which "environment variables" that can be used for switching. Our tools do so by only allowing dart.library.x.

[UNIcodeX]

Based on the above... I'm trying to build depending on the presence / absence of a dart file. The idea being that one branch has the file (pro version) and the other branch does not (free version), but I'm not doing something correctly. I have a file called isPro.dart in the same directory as another file which has the contents

import 'free.dart' if (isPro) 'pro.dart';

but that isn't working

Any ideas on what I can do?

If I test changing the condition to (dart.library.io) as in many examples, it imports the pro.dart contents, BUT, this isn't the condition I need checked.

In essence, I'm trying to work within the new spec to emulate old functionality. Any ideas??