Dash span bars broken with alignAboveContext

[Jay Anderson]

\version "2.15.41"

mus = \repeat unfold 5 c'1

\score
{
  \new StaffGroup \with
  { \override SpanBar #'glyph-name = #"dashed" }
  <<
    \new Staff="x"
    {
      \mus
      <<
        \mus
        \new Staff \with { alignAboveContext = "x" } \mus
      >>
    }
  >>
}

This example works in 2.14.2 with dash bars being between the staves.
In 2.15.41 the dashes are missing. Taking out the alignAboveContext
make dashed bars show, but the staff is below "x".

-----Jay