unsubscribe(event, listener) {
    this._assertSubscriptionParams(event, listener);
    if (this.listeners[BUILD_EVENT]) {
      this.listeners[BUILD_EVENT] = this.listeners[BUILD_EVENT].filter(item => item !== listener);
    }
  }
Unsubscribe from event. Currently only 'build' event is supported.
@param {String} event
@param {Function} listener