subscribe(event, listener) {
    this._assertSubscriptionParams(event, listener);
    this.listeners[BUILD_EVENT] = this.listeners[BUILD_EVENT] || [];
    this.listeners[BUILD_EVENT].push(listener);
    if (this.builded) {
      listener();
    }
  }
Subscribe to event. Currently only 'build' event is supported.
@param {String} event
@param {Function} listener