This is a DynamicScope that does not support any variables. It differs from
NoVarsDynamicScope in that it has hard failures for "backref" and "lastline"
accesses, since in the JRuby 1.3 cycle it was discovered that threads were
sharing a single dummyscope while still setting per-call backrefs. The hard
errors here are an attempt to catch such situations in the future, before
the optimizing compiler work makes such determinations in advance.