Private declarations show in extract and diff

[FMorschel]

Describe the bug
If you write a declaration in Dart where the name starts with _, it is private to that library and can't be accessed outside - we also have unnamed extensions but I've not checked those. They are showing in both diff and extract commands.

To Reproduce
Write a Dart file:

class _MyPrivateClass{}

// Maybe test these too
extension on int {
  String get foo => 'foo';
}

bool _foo => false;

You can't access either from outside this file (only with parts).

Actual and Expected behavior

For the private declarations not to show.

System info (please complete the following information):

• Dart SDK version: 3.9.0-edge.d3c323e26d1663ab422ff89b65d5a78b0e89c997 (main) (Sun Jul 6 22:55:04 2025 -0700) on "windows_x64"
• dart_apitool version 0.21.1