Source:

main() {
  barback.getAssetById(transformerAsset)
    .then((_) => transformerAsset)
    .catchError((e) => new AssetId(package, 'lib/$package.dart'),
          test: (e) => e is AssetNotFoundException);
}

Output:

main() {
  barback.getAssetById(transformerAsset).then((_) =>
      transformerAsset).catchError((e) => new AssetId(package, 'lib/$package.dart'),
      test: (e) => e is AssetNotFoundException);
}

I don't expect it to do as good a job line wrapping as I do, but at the very least it should stay within 80 columns.