code: function(oldKey, newKey) {
          var me = this,
              map = me.map,
              indexMap = me.indexMap,
              index = me.indexOfKey(oldKey),
              item;
              
          if (index > -1) {
              item = map[oldKey];
              delete map[oldKey];
              delete indexMap[oldKey];
              map[newKey] = item;
              indexMap[newKey] = index;
              me.keys[index] = newKey;
              me.generation++;
              
          }
      }
docs: Change the key for an existing item in the collection. If the old key
  does not exist this is a no-op.
  @param {Object} oldKey The old key
  @param {Object} newKey The new key