Declaration of worker:Worker stops application

If you have a simple Air mxml main file like so

<?xml version="1.0" encoding="utf-8"?>
<s:WindowedApplication xmlns:fx="http://ns.adobe.com/mxml/2009" 
                                                     xmlns:s="library://ns.adobe.com/flex/spark" 
                                                     xmlns:mx="library://ns.adobe.com/flex/mx">
          <fx:Declarations>
                    <!-- Place non-visual elements (e.g., services, value objects) here -->
          </fx:Declarations>
          <fx:Script>
                    <![CDATA[
                    ]]>
          </fx:Script>
</s:WindowedApplication>

You get an empty window.

However, if you have the same main mxml file like this, you get absolutely nothing, no errors or traces.

<?xml version="1.0" encoding="utf-8"?>
<s:WindowedApplication xmlns:fx="http://ns.adobe.com/mxml/2009" 
                                                     xmlns:s="library://ns.adobe.com/flex/spark" 
                                                     xmlns:mx="library://ns.adobe.com/flex/mx">
          <fx:Declarations>
                    <!-- Place non-visual elements (e.g., services, value objects) here -->
          </fx:Declarations>
          <fx:Script>
                    <![CDATA[
                              private var worker:Worker;
                    ]]>
          </fx:Script>
</s:WindowedApplication>

I'm not too sure why a declaration of a non referenced worker variable stops the app from showing itself (shouldn't it be optimized out and ignored when creating the bytecode ?).  I am using Flashbuilder 4.7 with the 3.5 SDK.