code: function(key, value) {
      var keys = this.keys,
          values = this.values,
          guid = guidFor(key);
  
      keys.add(key);
      values[guid] = value;
    }
docs: Adds a value to the map. If a value for the given key has already been
  provided, the new value will replace the old value.
  
  @method set
  @param {anything} key
  @param {anything} value