[shared_preferences_linux] Add support for Linux

[robert-ancell]

Description

Adds shared_preferences support for Linux.

Related Issues

Part of flutter/flutter#41720

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
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• I updated/added relevant documentation (doc comments with ///).
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• I updated CHANGELOG.md to add a description of the change.
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Breaking Change

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[robert-ancell]

I haven't worked out how to run test/shared_preferences_linux_test.dart - I tried flutter drive but couldn't work out how to get that work, either on this or path_provider_linux. All the different packages seem to have slightly different test layouts and I'm not sure what that "correct" method is.

The example program compiles and runs in Linux.

[stuartmorgan]

I haven't worked out how to run test/shared_preferences_linux_test.dart - I tried flutter drive but couldn't work out how to get that work, either on this or path_provider_linux.

Hm, https://github.com/flutter/plugins/blob/master/CONTRIBUTING.md#integration-tests should work. That's what the CI scripts are doing.

But the Linux CI run passed, so it looks like it's in good shape.

[robert-ancell]

Note I updated the example to use shared_preferences_linux directly instead of shared_preferences. I think that makes sense based on the ordering you've suggested!?

[stuartmorgan]

LGTM! I'll land this now, and publish it once post-submit is green.

[stuartmorgan]

It's live and ready to be endorsed!