[SkinPart] Metadata above private variable should result in compile error (currently results in runtime error)

Steps to reproduce:

// IconButton.mxml
<?xml version="1.0" encoding="utf-8"?>
<s:Button xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
xmlns:mx="library://ns.adobe.com/flex/halo"
skinClass="IconButtonSkin">

<fx:Script>
<![CDATA[
import spark.primitives.BitmapImage;
[SkinPart]
private var icon:BitmapImage;
]]
>
</fx:Script>
</s:Button>

// IconButtonSkin.mxml
<?xml version="1.0" encoding="utf-8"?>
<s:SparkSkin xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
minWidth="21" minHeight="21"
alpha.disabled="0.5">

<!-- host component -->
<fx:Metadata>
[HostComponent("IconButton")]
</fx:Metadata>

<!-- states -->
<s:states>
<s:State name="up" />
<s:State name="over" />
<s:State name="down" />
<s:State name="disabled" />
</s:states>

<s:BitmapImage id="icon"
left="5" verticalCenter="0" top="5" bottom="5" />

</s:SparkSkin>

1. Note that the [SkinPart] metadata is above a private variable in IconButton.
2. Launch the application.

Actual Results:

The following runtime error is reported:

ReferenceError: Error #1056: Cannot create property icon on IconButton.
at spark.components.supportClasses::SkinnableComponent/findSkinParts()[E:\dev\beta1\frameworks\projects\flex4\src\spark\components\supportClasses\SkinnableComponent.as:554]
at spark.components.supportClasses::SkinnableComponent/attachSkin()[E:\dev\beta1\frameworks\projects\flex4\src\spark\components\supportClasses\SkinnableComponent.as:524]
at spark.components.supportClasses::SkinnableComponent/validateSkinChange()[E:\dev\beta1\frameworks\projects\flex4\src\spark\components\supportClasses\SkinnableComponent.as:280]
at spark.components.supportClasses::SkinnableComponent/createChildren()[E:\dev\beta1\frameworks\projects\flex4\src\spark\components\supportClasses\SkinnableComponent.as:250]
at mx.core::UIComponent/initialize()[E:\dev\beta1\frameworks\projects\framework\src\mx\core\UIComponent.as:6510]

Expected Results:

Since the skin part needs to be public to be accessed by the skin, the compiler should throw an error saying [SkinPart] cannot be used for a private variable.

Workaround (if any):

Declare the [SkinPart] as public.