Wraps result to create a HitTestResult that implements the
BoxHitTestResult protocol for hit testing on RenderBoxes.
This method is used by RenderObjects that adapt between the
RenderBox-world and the non-RenderBox-world to convert a (subtype of)
HitTestResult to a BoxHitTestResult for hit testing on RenderBoxes.
The HitTestEntry instances added to the returned BoxHitTestResult are
also added to the wrapped result (both share the same underlying data
structure to store HitTestEntry instances).
BoxHitTestResult.wrap(super.result) : super.wrap();