How to filter build_runner input by extension?

Question

I am trying to make my custom builder using Dart's https://github.com/dart-lang/source_gen.

The buildExtensions in builder.dart is .dart

It builds for any dart file

buildExtensions = { '.dart': [generatedExtension]..addAll(additionalOutputExtensions) },

So even if in build.yaml, I set the build input to .source.dart

  functionUnwrap:
    import: 'package:my_generators/my_generators.dart'
    builder_factories: ['functionUnwrapBuild']
    auto_apply: dependents
    build_extensions: {".source.dart": [".unwrapped.dart"]}
    build_to: source
    applies_builders: ["source_gen"]

It ignores the ".source.dart" and still runs on any dart file.