code: function (key) {
        if (this.size === 0) {
          return;
        }
  
        var values = this._values;
        var guid = _emberMetalUtils.guidFor(key);
  
        return values[guid];
      }
docs: Retrieve the value associated with a given key.
  @method get
  @param {*} key
  @return {*} the value associated with the key, or `undefined`
  @private