Flat trills don't work in Lilypond 2.11

[Mark Nixon]

> I'm not top posting.

%% the standard solution for producing flat trills (which works in 2.10.33) no
longer works in 2.11 (seen in versions 2.11.14 + but also present in 2.11.34

\version "2.11.34"
\paper{ ragged-right=##t }


trillFlat = \markup { \musicglyph #"scripts.trill" \super \flat }

\relative c''' {
   \override TrillSpanner #'edge-text = #(cons trillFlat "" )
   a'1 ( \startTrillSpan |
   a2 ) \stopTrillSpan  a2 |
}