[flutter_analyzer_plugin] Migrate unit tests off package:test_reflective_loader - #193017

Removes the test_reflective_loader dev_dependency (and its transitive dart:mirrors dependency) from dev/flutter_analyzer_plugin, migrating all 18 test files to standard package:test test('...', () async { ... }) blocks.

Motivation

• package:test_reflective_loader relies on dart:mirrors to discover @reflectiveTest classes and invoke test_*() methods at runtime.
• package:analyzer_testing (AnalysisRuleTest -> PubPackageResolutionTest -> AbstractContextTest) has zero runtime dependency on test_reflective_loader—it only provides setUp() and tearDown() lifecycle hooks.
• Replacing defineReflectiveTests(...) and class-level test_*() methods with standard setUp / tearDown + test('...', () async { ... }) closures in main():
  • Eliminates test_reflective_loader (and dart:mirrors) from flutter/flutter's dependency tree (pubspec.lock / package_config.json), unblocking Dart 4 dart:mirrors removal.
  • Eliminates all // ignore: unreachable_from_main, non_constant_identifier_names lint suppressions across the test suite.
  • Keeps a 0 net line delta (+524 / -524 lines across 21 files).
  • Allows running dev/flutter_analyzer_plugin tests directly via flutter test dev/flutter_analyzer_plugin (previously blocked because flutter test disables dart:mirrors).

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• All existing and new tests are passing.