code: function SetConstructor(iterable) {
    if (IS_UNDEFINED(new.target)) {
      throw MakeTypeError(kConstructorNotFunction, "Set");
    }
  
    %_SetInitialize(this);
  
    if (!IS_NULL_OR_UNDEFINED(iterable)) {
      var adder = this.add;
      if (!IS_CALLABLE(adder)) {
        throw MakeTypeError(kPropertyNotFunction, adder, 'add', this);
      }
  
      for (var value of iterable) {
        %_Call(adder, this, value);
      }
    }
  }
docs: ------------------------------------------------------------------- Harmony Set