prefer_final_fields incorrectly warns in part of scenario #59219
Description
one.dart:
library one;
part "two.dart";
void foo() {
Foo foo = Foo._(1);
foo._foo = 3;
print(foo._foo);
}two.dart:
part of one;
class Foo {
Foo._(this._foo);
int _foo; // 🔥 incorrect `prefer_final_fields` warning
}Foo._foo in two.dart is being written to from one.dart, so it cannot be final and the lint should not trigger.