fix: add check for supertypes

[incendial]

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[X] Bug fix
[ ] New rule
[ ] Changes an existing rule
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

[github-actions]

Dart Code Metrics unused files report of dart_code_metrics. ✅

Summary

• Scanned package folders: bin, example, lib
• No unused files found! ✅

[github-actions]

Dart Code Metrics analyze report of dart_code_metrics. ✅

Summary

• Scanned folders: bin, example, lib, test

• Total scanned files: 415

• Total lines of source code: 7262

• Total classes: 283

• Average Cyclomatic Number per line of code: 0.35

• Average Source Lines of Code per method: 6

• Total tech debt: 1262.0 hours

• Found issues: 6 ⚠

[codecov]

Codecov Report

Merging #775 (2ce6d60) into master (c490ed9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 2ce6d60 differs from pull request most recent head b12ae56. Consider uploading reports for the commit b12ae56 to get more accurate results

@@           Coverage Diff           @@
##           master     #775   +/-   ##
=======================================
  Coverage   86.99%   87.00%           
=======================================
  Files         273      273           
  Lines        5784     5788    +4     
=======================================
+ Hits         5032     5036    +4     
  Misses        752      752           

Impacted Files	Coverage Δ
..._null_assertion/avoid_non_null_assertion_rule.dart	100.00% <ø> (ø)
...s/rules_list/avoid_non_null_assertion/visitor.dart	100.00% <100.00%> (ø)
lib/src/analyzer_plugin/analyzer_plugin.dart	0.00% <0.00%> (ø)
...ers/unused_code_analyzer/unused_code_analyzer.dart	98.75% <0.00%> (ø)
...ers/unused_l10n_analyzer/unused_l10n_analyzer.dart	90.00% <0.00%> (ø)

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