[vm, gc] Require a safepoint for heap iteration.

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
diff --git a/runtime/vm/scavenger.cc b/runtime/vm/scavenger.cc
index 32d62e000906025ed067aaa6032adfd2f326b1a0..6e34bd0fa65fabbc9fa9afaffb19fbf255f12812 100644
--- a/runtime/vm/scavenger.cc
+++ b/runtime/vm/scavenger.cc
@@ -731,6 +731,8 @@ void Scavenger::FlushTLS() const {
 }
 
 void Scavenger::VisitObjectPointers(ObjectPointerVisitor* visitor) const {
+  ASSERT(Thread::Current()->IsAtSafepoint() ||
+         (Thread::Current()->task_kind() == Thread::kMarkerTask));
   FlushTLS();
   uword cur = FirstObjectStart();
   while (cur < top_) {
@@ -740,6 +742,8 @@ void Scavenger::VisitObjectPointers(ObjectPointerVisitor* visitor) const {
 }
 
 void Scavenger::VisitObjects(ObjectVisitor* visitor) const {
+  ASSERT(Thread::Current()->IsAtSafepoint() ||
+         (Thread::Current()->task_kind() == Thread::kMarkerTask));
   FlushTLS();
   uword cur = FirstObjectStart();
   while (cur < top_) {