`avoid_unnecessary_return_variable` incorrectly triggers for variables used in multiple return paths

[xJac0b]

String test() {
  final someCondition = 1 == 1;
  final a = 'test';
  if (someCondition) return a;

  return 'something $a';
}

Expected behavior:
No lint - the variable a is used in multiple return statements, so it is not unnecessary.

Actual behavior:
The lint triggers on return a; incorrectly suggesting that the variable is only used in a single return.