code: function Listener(handler, once) {
    assert(typeof handler === 'function', '`handler` must be a function.');
    assert(typeof once === 'boolean', '`once` must be a function.');
    this.handler = handler;
    this.once = once;
  }
docs: Event Listener
  @constructor
  @ignore
  @param {Function} handler
  @param {Boolean} once
  @property {Function} handler
  @property {Boolean} once