Parse files with the language version the package declares

[maxim-sysoev]

Problem

getDartDirectivePackageNames calls parseString without a feature set, so it falls back to FeatureSet.latestLanguageVersion() — the newest language version the bundled analyzer knows about. With analyzer 13 that version is unreleased, and it rejects the final modifier on formal parameters:

void f(final int a) {}

Error parsing: ./lib/example.dart
Content produced diagnostics when parsed:
  extraneous_modifier: Can't have modifier 'final' here. - 1:8

The run aborts with exit code 1 before a single dependency is checked, on code the SDK analyzer accepts without complaint.

The behaviour arrived with 5.0.6 (Allow up to analyzer 13). The parsing code itself is byte-identical to 5.0.5; only the resolved analyzer changed — 5.0.5 resolves analyzer 12.1.0 and passes, 5.0.6 resolves analyzer 13.3.0 and fails.

Pinning the language version explicitly confirms the cause — the same snippet parses cleanly under every released version:

feature set	result
latestLanguageVersion()	fails
language 3.9 / 3.10 / 3.11 / 3.12	passes

Why it hits projects that never write final on a parameter

freezed emits the modifier itself. A declaration containing no final at all generates a constructor that does, so every generated file in a freezed project fails to parse. In our monorepo five of twelve packages turned red overnight with no source change — CI installs the tool unpinned, so 5.0.6 was picked up automatically.

Excluding generated files via dart_dependency_validator.yaml removes the parse errors but defeats the check: dependencies used only from generated code are then reported as unused, which has to be silenced in turn.

Fix

Derive the feature set from the package's SDK constraint — the same way the analyzer determines a package's language version — and fall back to the previous behaviour when the constraint is missing or unbounded. The feature set is computed once per package and passed into the parse; the parameter is optional, so the public function stays source-compatible.

Tests

Adds a regression test to executable_test.dart: a project whose library uses final on both a positional and a named parameter now validates cleanly. The test fails on master and passes with this change.

The rest of the suite is unchanged: 4 tests fail both with and without this change on my machine (+84 -4 either way), so they look unrelated to it.

Suppressing diagnostics with throwIfDiagnostics: false was considered and rejected: it would also hide genuinely malformed files. With this change a truly broken file still reports expected_token diagnostics as before.