loader(_) {
      if (arguments.length) {
        this._loader = _;
        return this;
      } else {
        return this._loader;
      }
    }
Gets or sets the loader instance to use for data file loading. A
loader object must provide a "load" method for loading files and a
"sanitize" method for checking URL/filename validity. Both methods
should accept a URI and options hash as arguments, and return a Promise
that resolves to the loaded file contents (load) or a hash containing
sanitized URI data with the sanitized url assigned to the "href" property
(sanitize).
@param {object} _ - The loader instance to use.
@return {object|Dataflow} - If no arguments are provided, returns
  the current loader instance. Otherwise returns this Dataflow instance.