StressMethodComparator is not thread-safe

It's an AllStatic class (not declared that way but all functions and fields are static). If you run parallel_class_loading tests with this stress option, they crash with:

#
# Internal Error (/java/east/u1/cphillim/hg/rt.rewriter/src/share/vm/runtime/handles.inline.hpp:109), pid=22476, tid=2401262448
# assert(h._thread == Thread::current()) failed: thread must be current


I don't know if this had a lock around it that got removed with code rearrangement. Making MethodComparator members instance objects would probably fix this.