code: function _getMixinInstance(methodName) {
      if (this instanceof Mixin) return this;
      var instanceKeys = this.constructor[INSTANCE_PROPERTIES_MAP],
          mixinProp = instanceKeys[methodName],
          mixin = this[mixinProp];
      return mixin;
      // if (!mixin) throw new Error('Mixin ' + mixinProp + ' does not exist');
      // return mixin;
  }
docs: Returns the reference to the instance of mixin subclass.
  This method is used when methods are exposed on the host class prototype (using addMehtods) rather than on host instance.
  Subclasses should not use this methods - whenever subclass method is exposed on the prototype it will be wrapped to set correct context for the subclass method.
  
  @private
  @return {Object}