The IBorder interface defines the interface that all classes
used for border skins should implement.
It is not an error if the border skin does not implement IBorder.
In this case, however, the container using the skin cannot determine
the border metrics of the border.
Therefore, the container places content starting at its top-left edge
(adjusted for padding, if any).
For the HaloBorder class, the borderThickness style
usually determines the value of the borderMetrics style.
For graphical skin classes, Flex examines the scale9Grid
property to determine the value of the borderMetrics style.
Returns an EdgeMetrics object for the border that has four properties:
left, top, right,
and bottom.
The value of each property is equal to the thickness of one side
of the border, in pixels.
Implementation public function get borderMetrics():EdgeMetrics
