... the maMarkStack, which causes the order of calls to
startFastElement()/endFastElement() to differ from the order of the tags
that are written into the output.

This is an attempt to improve the assertions, but if an assertion fails
it's generally not obvious where the problem actually is since the
unpredictable order may cause the problem to be detected and reported
much later than its root cause.

Let's see if this finds any new problems in export testing.

Change-Id: I97699cc8ef9b18ea9f4f221d5210134feecf0336