walkAsync now does not assume `path` is a directory now.

walk.js

/*
  Recursively traverses the given dir and returns a promise that resolves to
  a list of files.

  The promise is resolved only after all sub-directories being traversed.
*/

var walkAsync = function(path) {
  return statAsync(path).then(function(stat){
    if(stat.isDirectory()){
      return readdirAsync(path).then(function(files){
        return Promise.all(files.map(function(file){
          return walkAsync(path + "/" + file);
        }));
      });
    }else{
      return Promise.resolve([path])
    }
  }).then(function(files){
    return [].concat.apply([], files)
  });
};

/* Utilities */

function readdirAsync(path){
  var fs = fs || require('fs');
  return new Promise(function(resolve, reject){
    fs.readdir(path, function(err, files){
      if(err) {reject(err);}
      resolve(files);
    });
  });
}

function statAsync(path){
  var fs = fs || require('fs');
  return new Promise(function(resolve, reject){
    fs.stat(path, function(err, stats){
      if(err) {reject(err);}
      resolve(stats);
    });
  });
}