code: function getCutoffLength(arr, cutoff) {
          //because AsyncQueue#then could add sync task at any time
          // we must read directly from this.tasks.length
          var length = arr.length;
          if (~cutoff && cutoff < length) length = cutoff;
          return length;
      }
docs: @private
  @name getCutoffLength
  Get cut-off length for iteration
  
  @param {Array}  arr
  @param {Number} cutoff