In current ES5 (all mobile browsers and all desktops but IE < 9) is possible to somehow simulate Harmony Symbol constructor.

gistfile1.js

var Symbol;
if (!Symbol) {
  Symbol = (function(Object){
    var defineProperty = Object.defineProperty,
        prefix = '__simbol' + Math.random() + '__',
        id = 0;
    function Symbol() {
      var self = this;
      defineProperty(
        Object.prototype,
        self.__symbol__ = prefix + id++,
        {
          enumerable: false,
          configurable: true,
          set: function set(value) {
            defineProperty(this, self.__symbol__, {
              enumerable: false,
              configurable: true,
              writable: true,
              value: value
            });
          }
        }
      );
    }
    defineProperty(Symbol.prototype, 'toString', {
      enumerable: false,
      writable: false,
      configurable: false,
      value: function toString() {
        return this.__symbol__;
      }
    });
    return Symbol;
  }(Object));
}

// example
var sym = new Symbol;
var o = {};
o[sym] = 123;
console.log(o[sym]);

blog entry about Symbols

for a more complete/updated polyfill please see this repository