Fix simulator with background compilation on.

runtime/vm/longjump.cc

Index: runtime/vm/longjump.cc
diff --git a/runtime/vm/longjump.cc b/runtime/vm/longjump.cc
index ef141c9b220c07d42e43f43e75d20fa5d51d278f..13e01337c53829c2af4413991955162e9a4ffb5e 100644
--- a/runtime/vm/longjump.cc
+++ b/runtime/vm/longjump.cc
@@ -29,7 +29,10 @@ bool LongJumpScope::IsSafeToJump() {
   uword jumpbuf_addr = Isolate::GetCurrentStackPointer();
 #if defined(USING_SIMULATOR)
   Simulator* sim = Simulator::Current();
-  uword top_exit_frame_info = sim->top_exit_frame_info();
+  // When using simulator, only mutator thread should refer to Simulator
+  // since there can be only one per isolate.
+  uword top_exit_frame_info = thread->IsMutatorThread() ?
+      sim->top_exit_frame_info() : 0;
 #else
   uword top_exit_frame_info = thread->top_exit_frame_info();
 #endif