Sample async split of control structure for futures

test.dart

http://goo.gl/xDA15
I don't like nesting either, but I think trying to hide your flow control by writing a branching set of logic as a 'pipeline' is a mistake.
I would refactor into methods, like this (again, guessing at what you're trying to do):

main() => getEntries("wibble.txt");

/// Reads the files listed in [filename] and returns their contents as strings
/// or null if [filename] doesn't exist
Future<List<String>> getEntries(String filename) {
  final file = new File(filename);
  return file.exists().chain((e) => e
      ? new Future.immediate(null)
      : file.readAsText().transform(splitLine).chain(readFiles));
}

Future<List<String>> readFiles(List<String> files) =>
    Futures.wait(files.map((f) => new File(f).readAsText());

List<String> splitLines(String text) => text.split("\n");