Skin Not Being Sized Correctly

In this application

```

```

where Collapsible Panel is

```

```

And CollapsiblePanelSkin is

```

[HostComponent("components.CollapsiblePanel")]

```

The button should be spread to the width of the screen (as it does in Flex). Instead, it's got a 70px width.

Reproduce the issue using the Application, CollapsiblePanel, and CollapsiblePanelSkin MXML snippets from the report. Start by inspecting how left and right constraints are handled for the Button inside SparkSkin, then compare the result with the expected full-width behavior. Done means the button spans the available screen width as described.

Written by the indexing model from the issue text.