The ASDocs are just HTML and there is not an explicit component like that in Flex.  I assume the 'window' on the ASDocs is some form of AJAX but haven't reviewed code.  
But, it should be easy to create one.  
<mx:VBox>
 <mx:TextInput id="input" visible="true" />
 <mx:Button id="expandCollapseButton" visible="true" label=">" click="onClick()" />
</mx:VBox>
In your onClick method do something like this: 
public function onClick():void{
 if(this.expandCollapseButton.label == '>'){
  this.input.visible=false;
  this.input.includeInLayout=false;
  this.expandCollapseButton.label = "<";
 } else {
  this.input.visible=true;
  this.input.includeInLayout=true;
  this.expandCollapseButton.label = ">";
 }
}
States, as mentioned in a comment to your question may be another way to implement this.  I'm sure there are others.  
I would think hard about what you're trying to accomplish.  I find the yellow box non-intuitive and very annoying.  I hav to click a "play" button to hide it.  "Play" usually means goto and for the longest time I would not click it because I didn't want to leave the page.  
Disclaimer: Code written in browser