code: function waitForCancel() {
    /* jshint validthis: true */
    var self = this;
    var cancel_batch = {};
    cancel_batch[grpc.opType.RECV_CLOSE_ON_SERVER] = true;
    self.call.startBatch(cancel_batch, function(err, result) {
      if (err) {
        self.emit('error', err);
      }
      if (result.cancelled) {
        self.cancelled = true;
        self.emit('cancelled');
      }
    });
  }
docs: Wait for the client to close, then emit a cancelled event if the client
  cancelled.
  @private