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