Run the following under 4.7 and master:

#include <QtCore/QDir>
#include <QDebug>

int main(int argc, char *argv[])
{
QDir dir(QLatin1String(":/prefix/"));
qDebug() << dir.absoluteFilePath(QLatin1String("foo.bar"));
return 0;
}

Under 4.7 I get the expected:
":/prefix/foo.bar"

Under master I get buggy:
"/home/jkobus/test/absoluteFilePath-build-desktop/:/prefix/foo.bar"