Fixed Dart compilation issue.

[matux]

Description of the change

Fixed an obscure compilation issue that arose while working on the current features. The error thrown by the analyzer/compiler is neither correct nor particularly precise, but it led to an improvement in how we express persistable records, which fixed the complain.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Maintenance
• New release

Related issues

• Fix Surfaced generics-related compilation error #76.

Checklists

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• All tests related to the changed code pass in development

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• "Ready for review" label attached to the PR and reviewers assigned
• Issue from task tracker has a link to this pull request
• Changes have been reviewed by at least one other engineer

[cyrusradfar]

Appreciate the attention to detail in the error handling, Matias. Thank you, approved.