function ls_(successCallback) {
    if (!fs_) {
      return;
    }

    var entries = [];
    var reader = cwd_.createReader();

    var readEntries = function() {
      reader.readEntries(function(results) {
        if (!results.length) {
          entries = entries.sort();
          successCallback(entries);
        } else {
          entries = entries.concat(util.toArray(results));
          readEntries();
        }
      }, errorHandler_);
    };

    readEntries();
  }
Read contents of current working directory. According to spec, need to
keep calling readEntries() until length of result array is 0. We're
guaranteed the same entry won't be returned again.

@param {Function} successCallback Function to call when listing is done