function getPlugin(instance, pluginName) {
  if (typeof pluginName !== 'string') {
    throw Error('Only strings can be passed as "plugin" parameter');
  }

  var _pluginName = (0, _string.toUpperCaseFirst)(pluginName);

  if (!registeredPlugins.has(instance) || !registeredPlugins.get(instance)[_pluginName]) {
    return void 0;
  }

  return registeredPlugins.get(instance)[_pluginName];
}
@param {Object} instance
@param {String|Function} pluginName
@returns {Function} pluginClass Returns plugin instance if exists or `undefined` if not exists.