code: function(index) {
      var calls = this.all();
      var call = calls[index];
  
      return call ? call.object : null;
    }
docs: Get the this that were passed to a specific invocation of this spy.
  
  @name Spy#calls#thisArgFor
  @function
  @param {Integer} index The 0-based invocation index.
  @return {Object}