test: fix tests in dart 2.19

[amondnet]

Describe the changes proposed in this Pull Request.

If the PR fixes a specific issue, reference the issue with Fixes #.

Dart 2.19 doesn't delegate inaccessible private names to noSuchMethod.

https://github.com/dart-lang/sdk/blob/main/CHANGELOG.md#2190
dart-lang/language#2020
dart-lang/sdk#49687
dart-lang/site-www#4263

https://github.com/mobxjs/mobx.dart/blob/master/mobx/test/observable_test.dart

The test fails on dart 2.19.

https://github.com/mobxjs/mobx.dart/actions/runs/3262020308/jobs/5359681093

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Pull Request Checklist

• If the changes are being made to code, ensure the version in pubspec.yaml is updated.
• Increment the major/minor/patch/patch-count, depending on the complexity of change
• Add the necessary unit tests to ensure the coverage does not drop
• Update the Changelog to include all changes made in this PR, organized by version
• Run the melo run set_version command from the root directory
• Include the necessary reviewers for the PR
• Update the docs if there are any API changes or additions to functionality

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[codecov]

Codecov Report

Merging #879 (5f7d132) into master (021f4a0) will decrease coverage by 1.04%.
The diff coverage is 100.00%.

❗ Current head 5f7d132 differs from pull request most recent head c6e238c. Consider uploading reports for the commit c6e238c to get more accurate results

@@             Coverage Diff             @@
##            master     #879      +/-   ##
===========================================
- Coverage   100.00%   98.95%   -1.05%     
===========================================
  Files            5       55      +50     
  Lines           97     1923    +1826     
===========================================
+ Hits            97     1903    +1806     
- Misses           0       20      +20     

Flag	Coverage Δ
flutter_mobx	100.00% <ø> (ø)
mobx	98.53% <100.00%> (?)
mobx_codegen	100.00% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mobx/lib/src/core.dart	100.00% <ø> (ø)
mobx/lib/src/core/atom.dart	100.00% <100.00%> (ø)
mobx/lib/src/core/computed.dart	100.00% <100.00%> (ø)
mobx/lib/src/core/context.dart	97.16% <100.00%> (ø)
mobx/lib/src/core/reaction.dart	98.52% <100.00%> (ø)
mobx/lib/src/core/action.dart	100.00% <0.00%> (ø)
mobx/lib/src/utils.dart	100.00% <0.00%> (ø)
mobx_codegen/lib/src/type_names.dart	100.00% <0.00%> (ø)
mobx_codegen/lib/src/template/store.dart	100.00% <0.00%> (ø)
...src/api/extensions/primitive_types_extensions.dart	0.00% <0.00%> (ø)
... and 45 more

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[fzyzcjy]

LGTM except the codecov

[tim-eucalyptus]

Would be great to see this merged as it's holding up: #877. Thanks!

[tim-eucalyptus]

Hey can someone please merge this already? It's really holding up our project as we cannot update to freezed 2.2.1 with a critical bug fix.

[tim-eucalyptus]

Any update on this??

[HudsonAfonso]

news?