docs: 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;
  }