Returns the stats for a file, or null if the file does not exist

@param filename
@returns {fs.Stat}
@async
function(filename) {
      return new Promise((resolve, reject) => {
        fs.stat(filename, function(err, stats) {
          if (err && err.code != "ENOENT") {
            reject(err); 
          } else {
            resolve(err ? null : stats);
          }
        });
      });
    }