code: function exists(path, callback) {
          stat(path, function (err) {
              if (err) {
                  if (err === FileSystemError.NOT_FOUND) {
                      callback(null, false);
                  } else {
                      callback(err);
                  }
                  return;
              }
  
              callback(null, true);
          });
      }
docs: Determine whether a file or directory exists at the given path by calling
  back asynchronously with either a FileSystemError string or a boolean,
  which is true if the file exists and false otherwise. The error will never
  be FileSystemError.NOT_FOUND; in that case, there will be no error and the
  boolean parameter will be false.
  
  @param {string} path
  @param {function(?string, boolean)} callback