An alternative, less memory greedy, more robust WeakMap polyfill.

WeakMap.js

var WeakMap = WeakMap || (function (Object) {

  /*! (C) Andrea Giammarchi - Mit Style License */

  var
    dP = Object.defineProperty,
    hOP = Object.hasOwnProperty,
    WeakMapPrototype = WeakMap.prototype,
    i = 0
  ;

  function WeakMap() {'use strict';
    this.clear();
  }

  WeakMapPrototype.clear = function clear() {
    this.name = '__WeakMap#'.concat(++i, Math.random(), '__');
  };

  WeakMapPrototype['delete'] = function del(key) {
    return this.has(key) && delete key[this.name];
  };

  WeakMapPrototype.get = function get(key) {
    return this.has(key) ? key[this.name] : void 0;
  };

  WeakMapPrototype.has = function has(key) {
    return hOP.call(key, this.name);
  };

  WeakMapPrototype.set = function set(key, value) {
    if (this.has(key)) {
      key[this.name] = value;
    } else {
      dP(key, this.name, {
        writable: true,
        configurable: true,
        value: value
      });
    }
    return this;
  };

  return WeakMap;

}(Object));

full test coverage for what it's provided

var wm = new WeakMap;
var o = {};
var d = {};
console.assert([
  !wm.has(o),
  wm.get(o) === undefined,
  wm.set(o, d) && wm.has(o),
  wm.get(o) === d,
  wm.clear() || !wm.has(o),
  wm.set(d, o) && wm.has(d),
  wm.set(o, d) && wm.has(o),
  wm.get(d) === o,
  wm.delete(o),
  !wm.delete(o),
  wm.delete(d),
  !wm.delete(d),
  !wm.has(o),
  !wm.has(d)
].every(Boolean));