addXFileEdit with customPath does not save file

[unitmdev]

Describe the bug

Applying a DartFix to a customPath does not save the file. Reading the file later with File(path).readAsStringSync() ignores any fixes applied using addXFileEdit until customPath is saved. All unsaved changes are ignored when reading file contents.

To Reproduce

class SomeFix extends DartFix {
  @override
  void run(
    CustomLintResolver resolver,
    ChangeReporter reporter,
    CustomLintContext context,
    AnalysisError analysisError,
    List<AnalysisError> others,
  ) {
    context.registry.addNode((node) {
      final file = File('${Directory.current.path}/lib/foo.txt');
      final fileContents = file.readAsStringSync();

      if (fileContents == '') {
        reporter
            .createChangeBuilder(
              message: 'Fill empty file',
              priority: 0,
            )
            .addGenericFileEdit(
              (builder) => builder.addSimpleInsertion(0, 'foo'),
              customPath: file.path,
            );
      }
    });
  }
}

Expected behavior

Applying SomeFix should only be possible once. After foo is added to the file, the ChangeBuilder shouldn't get created.

I noticed that the analyzer plugin gets file contents from ResourceProvider, but I don't see how to access that. There must be a more reliable way to read the file contents than File(path).readAsStringSync(), or there must be a way to save the customPath file.