FED-5022 Drop Dart 2 support

[greglittlefield-wf]

Motivation

Maintaining a large range of analyzer versions comes with a bit of overhead, and getting to the latest analyzer versions (#1012) while also supporting Dart 2 just isn't feasible.

We've supported Dart 2 + 3 in the same version of over_react for a while, and now it's time to drop Dart 2 support.

Changes

• Update CI to no longer run on Dart 2, remove conditional Dart-2-only/Dart-3-only logic
• Raise Dart SDK lower bound from >=2.19 to >=3.0.0
• Fix errors/warnings resulting from upping the language version to 3.0 via the upped SDK constraint

  • class-used-as-mixin errors. From CHANGELOG.md:

    • Fix Dart language level 3.0 errors and downstream compile errors by switching internal classes to mixin declarations

      • GeneratedClass - switched to mixin

      • PropsMapViewMixin, StateMapViewMixin, MapViewMixin - switched to mixin, also moved implements to on

      • Hand-authored mixin "generated" classes:

        • Core: $AriaPropsMixin, $ConnectFluxPropsMixin, $CssClassPropsMixin, $DomPropsMixin, $ReactPropsMixin, $SvgPropsMixin, $UbiquitousDomPropsMixin
        • Components: $ErrorBoundaryPropsMixin, $ErrorBoundaryStateMixin, $ResizeSensorPropsMixin, $TransitionPropsMixin

      • While some of these are technically a breaking changes, there's very little chance of them impacting consumers, so we're releasing it as a minor.

        These classes have no known usages outside of over_react or its generated code, and are for internal use only.

        Additionally, the *MapViewMixin ones never should have been exported in the first place, and have been deprecated accordingly.

  • Remove unreachable switch statement default cases

  • Use modern equivalent to implicit-casts flag in analysis_options.yaml
    have ever been exported

• Add regression tests for props mixin compile errors I didn't discover until pulling this branch into an internal Dart 3 consumer: test/over_react/component/props_mixin_compile_test.dart
• Delete all test cases for non-null-safe and unsound null safety consumer code
  • Everything must be null-safe in Dart 3, so these are no longer applicable and wouldn't compile even if we tried to run them
• Add changelog entry

QA steps

• CI passes
• Verify that new test/over_react/component/props_mixin_compile_test.dart regression tests fail on the commit before the fix (35d57cb), and pass on latest commit
• Override a big Dart 3 app with this branch and verify it compiles cleanly

[greglittlefield-wf]

This is kind of arbitrary; I just picked an older-but-not-too-old version of Dart 3

[robbecker-wf]

Looks great!

[robbecker-wf]

QA +1 CI passes. Builds and runs in our internal app.

[greglittlefield-wf]

@Workiva/release-management-p

[rmconsole-wf]

+1 from RM