src/emitter.js
function once(eventName, handler, context, args) {
    this.on(eventName, handler, context, args, true);
}
function once(eventName, handler, context, args) {
    this.on(eventName, handler, context, args, true);
}
function once ( eventName , handler , context , args ) { this . on ( eventName , handler , context , args , true ) ; }
Adds listener for an event that should be called once
@param {string} eventName
@param {function} handler
@param {object} context
@param {array} args
Adds listener for an event that should be called once