[Tool] `plugins_test.dart` use of `expect(registrantImpl, isNot(...))` always passes

```dart
final File registrantImpl = linuxProject.managedDirectory.childFile(
  'generated_plugin_registrant.cc',
);

expect(registrantImpl, exists);
expect(registrantImpl, isNot(contains('SomePlugin')));
expect(registrantImpl, isNot(contains('some_plugin')));
```

This does not do what you expect ... it always silently passes (and does not read the file).