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

runtime/vm/scavenger.cc

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/scavenger.h"
 
 #include "vm/dart.h"
 #include "vm/dart_api_state.h"
 #include "vm/isolate.h"
 #include "vm/lockers.h"
@@ skipping 713 matching lines @@
 
 void Scavenger::FlushTLS() const {
   ASSERT(heap_ != NULL);
   if (heap_->isolate()->IsMutatorThreadScheduled()) {
     Thread* mutator_thread = heap_->isolate()->mutator_thread();
     mutator_thread->heap()->new_space()->set_top(mutator_thread->top());
   }
 }
 
 void Scavenger::VisitObjectPointers(ObjectPointerVisitor* visitor) const {
+  ASSERT(Thread::Current()->IsAtSafepoint() ||
+         (Thread::Current()->task_kind() == Thread::kMarkerTask));
   FlushTLS();
   uword cur = FirstObjectStart();
   while (cur < top_) {
     RawObject* raw_obj = RawObject::FromAddr(cur);
     cur += raw_obj->VisitPointers(visitor);
   }
 }
 
 void Scavenger::VisitObjects(ObjectVisitor* visitor) const {
+  ASSERT(Thread::Current()->IsAtSafepoint() ||
+         (Thread::Current()->task_kind() == Thread::kMarkerTask));
   FlushTLS();
   uword cur = FirstObjectStart();
   while (cur < top_) {
     RawObject* raw_obj = RawObject::FromAddr(cur);
     visitor->VisitObject(raw_obj);
     cur += raw_obj->Size();
   }
 }
 
 void Scavenger::AddRegionsToObjectSet(ObjectSet* set) const {
@@ skipping 166 matching lines @@
   }
 
   Scavenge();
 
   // It is possible for objects to stay in the new space
   // if the VM cannot create more pages for these objects.
   ASSERT((UsedInWords() == 0) || failed_to_promote_);
 }
 
 }  // namespace dart