startUp method
Emits lints for a given file.
run will only be invoked with files respecting filesToAnalyze
Implementation
@override
Future<void> startUp(
  CustomLintResolver resolver,
  CustomLintContext context,
) async {
  // Relying on shared state to execute all linters in a single AstVisitor
  if (context.sharedState.containsKey(_stateKey)) return;
  context.sharedState[_stateKey] = Object();
  final unit = await resolver.getResolvedUnitResult();
  context.addPostRunCallback(() {
    final linterVisitor = LinterVisitor(context.registry.nodeLintRegistry);
    unit.unit.accept(linterVisitor);
  });
}