Document (current) best practices for platform-specific code (PSC)

[kevmoo]

We have basic support for PSC, but it's not well documented and there are potentially big holes that a user could fall through.

• Document the recommended approach here
• Link to working examples (pkg:http, etc)
• Explain pitfalls (defining types, exports, etc)
• Hint at future direction?

[banalg]

If I can just add my two cents on this topic. I'm currently developing some IoT stuff on a RaspberryPi with Dart. I usually test on my laptop where it isn't possible to use the physicals pins of the Raspberry Pi, so I developed a mocked library to simulate button press and on/off a led. But I currently need to manually switch the package import between the real and the mocked GPIO library.
I checked the http library and the control seems be done by checking the presence of these dart components : dart.library.html or dart.library.io.
But in my case, as both (Raspberry Pi and my laptop) use the dart.io library, which kind of control could I use ? Isit possible to check the operating system, or another things ?

[natebosch]

Is it possible to check the operating system, or another things ?

No, only the presence or absence of the dart: libraries. Imports based on arbitrary other information are not compatible with our approach for modular builds.

[escamoteur]

@mit-mit Another application would be see here flutter/flutter#30845 if someone want's to have different pages for iOS/Android but don't want to carry the code of the not used platform with it.

[mit-mit]

Closing as a dupe of #1569, which has an active PR. If anything is missing there, please comment on the PR