CPLAT-6138: Update handler chain utils for dart 2.4 compat

[evanweible-wf]

Closes #304

CPLAT-6138

Motivation

When Dart 2.4 is released (imminent), it comes with a small breaking language change that unfortunately affects the handler chaining utils in this package. @greglittlefield-wf and @georgelesica-wf were able to update them in such a way that works around the breaking change without adversely affecting existing usages.

Changes

• The CallbackUtil class no longer uses its generic type param (that previously was expected to extend Function) and instead explicitly marks the relevant types in the class body as covariant Function to achieve the same typing.
• The CallbackUtilNArg classes no longer specify a type param when extending CallbackUtil since it is unused.
• Run CI on the dart dev channel as well as stable to hopefully illuminate issues like this as early as possible.

Fixes #304

Release Notes

Review

See CONTRIBUTING.md for more details on review types (+1 / QA +1 / +10) and code review process.

Please review:
@greglittlefield-wf @georgelesica-wf @aaronlademann-wf

QA Checklist

• Tests were updated and provide good coverage of the changeset and other affected code
• Manual testing was performed if needed
  • Push up a consumer test PR of this branch in WSD and verify that it passes

Merge Checklist

While we perform many automated checks before auto-merging, some manual checks are needed:

• A Client Platform member has reviewed these changes
• There are no unaddressed comments - this check can be automated if reviewers use the "Request Changes" feature
• For release PRs - Version metadata in Rosie comment is correct

[aviary2-wf]

Security Insights

No security relevant content was detected by automated scans.

Action Items

• Review PR for security impact; comment "security review required" if needed or unsure
• Verify aviary.yaml coverage of security relevant code

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Questions or Comments? Reach out on Slack: #support-infosec.

[greglittlefield-wf]

+1 to @evanweible-wf's commits

[aaronlademann-wf]

+1

[evanweible-wf]

QA +1

• CI passes
• Dart2 portions of WSD consumer test passed: https://github.com/Workiva/web_skin_dart/pull/1233

[evanweible-wf]

+1 to @greglittlefield-wf's commits

[evanweible-wf]

@Workiva/release-management-p