code: function(plugins) {
  		var p = this._plugins;
  		([]).concat(plugins).forEach(function(plugin) {
  			if (p.indexOf(plugin) === -1) {
  				p.push(plugin);
  			}
  		});
  
  		this._cacheId++;
  	}
docs: Registers the given plugin(s) if not already registered.
  @param {IPlugin[]|IPlugin} plugins plugin instance(s).