checkMessageHandler method Null safety
Returns true if the
handler argument matches the
passed to setMessageHandler.
This method is useful for tests or test harnesses that want to assert the handler for the specified channel has not been altered by a previous test.
Passing null for the
handler returns true if the handler for the
channel is not set.
@override bool checkMessageHandler(String channel, MessageHandler? handler) => _handlers[channel] == handler;