auto merge of #9742 : alexcrichton/rust/issue-9739, r=brson

This changes an `assert_once_ever!` assertion to just a plain old assertion
around an atomic boolean to ensure that one particular runtime doesn't attempt
to exit twice.

Closes #9739

diff --cc src/libstd/rt/mod.rs
index c7c4d3fc4f6256d93518dc716fd7c24acaf5051e,fa9d767ec3fc9b5350243382dd3ad8facea738b3..2db973b602ee87243c662d5f7cf6acf2ad53b2cd
--- 1/src/libstd/rt/mod.rs
--- 2/src/libstd/rt/mod.rs
+++ b/src/libstd/rt/mod.rs
@@@ -67,8 -67,10 +67,8 @@@ use rt::local::Local
  use rt::sched::{Scheduler, Shutdown};
  use rt::sleeper_list::SleeperList;
  use rt::task::{Task, SchedTask, GreenTask, Sched};
 -use rt::thread::Thread;
 -use rt::work_queue::WorkQueue;
  use rt::uv::uvio::UvEventLoop;
- use unstable::atomics::{AtomicInt, SeqCst};
+ use unstable::atomics::{AtomicInt, AtomicBool, SeqCst};
  use unstable::sync::UnsafeArc;
  use vec;
  use vec::{OwnedVector, MutableVector, ImmutableVector};