code: function( state, action ) {
  	if ( typeof state[ action.siteId ] !== 'undefined' ) {
  		return Object.assign( {}, state, {
  			[ action.siteId ]: pluginsForSite( state[ action.siteId ], action ),
  		} );
  	}
  	return state;
  }
docs: /*
  Helper function to update a plugin's state after a successful plugin action
  (multiple action-types are possible)