docs: function (key, value) {
        var keys = this._keys;
        var values = this._values;
        var guid = _emberMetalUtils.guidFor(key);
  
        // ensure we don't store -0
        var k = key === -0 ? 0 : key;
  
        keys.add(k, guid);
  
        values[guid] = value;
  
        this.size = keys.size;
  
        return this;
      }