docs: function(type, handler) {
        if (!this.listeners_)
          this.listeners_ = Object.create(null);
        if (!(type in this.listeners_)) {
          this.listeners_[type] = [handler];
        } else {
          var handlers = this.listeners_[type];
          if (handlers.indexOf(handler) < 0)
            handlers.push(handler);
        }
      }