MessageChannel(String name, { bool sync: false })

If the channel of name is cached, return the cached one, if not, return a new one. argument sync will only take effect when the channel is non-cached

Source 

factory MessageChannel(String name, {bool sync: false}) {
  if (!_channels.containsKey(name))
    return new MessageChannel._init(name, sync: sync);
  return _channels[name];
}