code: function Runnable(title, fn) {
    this.title = title;
    this.fn = fn;
    this.async = fn && fn.length;
    this.sync = ! this.async;
    this._timeout = 2000;
    this._slow = 75;
    this.timedOut = false;
  }
docs: Initialize a new `Runnable` with the given `title` and callback `fn`.
  
  @param {String} title
  @param {Function} fn
  @api private