src/shared/index.js/on
export function on ( eventName, handler ) {
	if ( eventName === 'teardown' ) return this.on( 'destroy', handler );

	var handlers = this._handlers[ eventName ] || ( this._handlers[ eventName ] = [] );
	handlers.push( handler );

	return {
		cancel: function () {
			var index = handlers.indexOf( handler );
			if ( ~index ) handlers.splice( index, 1 );
		}
	};
}