Please explain the magic! (Question)

Question

The ActionScript snippet below is from the BlaseDS chat sample app. Can someone please explain the magic that declares the chatMessage property of AsyncMessage.body (IMessage.body?) object? It's not in the docs anywhere so I'm guessing it is not built into AsyncMessage. And it's not defined in any of the sample app source files.
Coming from a strongly-typed development world, seeing a property that apparently has no declaration and is not explicitly instantiated does not pass the sniff-test.
Thanks.
<mx:Script>
  <![CDATA[
   import mx.messaging.messages.AsyncMessage;
   import mx.messaging.messages.IMessage;
   private function send():void
    var message:IMessage = new AsyncMessage();
    message.body.chatMessage = msg.text;
    producer.send(message);
    msg.text = "";
   private function messageHandler(message:IMessage):void
    log.text += message.body.chatMessage + "\n";
  ]]>

Answer 1

Hold your nose, because that is the dynamic "feature" of Actionscript.  Pretty much every class derives from Object (http://livedocs.adobe.com/flash/9.0/ActionScriptLangRefV3/Object.html) which is a dictionary of key-value property pairs.  The {"chatMessage", msg.text as Object} pair is created upon assignment.
I am fairly new to Flex/AS, and while not having to declare types is convenient for quick and dirty coding, I try to avoid it in production code...I have been bitten by refactoring a class and not catching all the places where it was referenced dynamically.  I'm sure there are comprehensive pro and con arguments out there.