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

runtime/vm/pages.cc

Index: runtime/vm/pages.cc
diff --git a/runtime/vm/pages.cc b/runtime/vm/pages.cc
index 895d2b2439878d29e023f9e705cc918890b98eed..4f62c59d6f0ea02bef7f2ec2e98a7d60f510489e 100644
--- a/runtime/vm/pages.cc
+++ b/runtime/vm/pages.cc
@@ -113,6 +113,7 @@ void HeapPage::Deallocate() {
 }
 
 void HeapPage::VisitObjects(ObjectVisitor* visitor) const {
+  ASSERT(Thread::Current()->IsAtSafepoint());
   NoSafepointScope no_safepoint;
   uword obj_addr = object_start();
   uword end_addr = object_end();
@@ -125,6 +126,7 @@ void HeapPage::VisitObjects(ObjectVisitor* visitor) const {
 }
 
 void HeapPage::VisitObjectPointers(ObjectPointerVisitor* visitor) const {
+  ASSERT(Thread::Current()->IsAtSafepoint());
   NoSafepointScope no_safepoint;
   uword obj_addr = object_start();
   uword end_addr = object_end();
@@ -752,9 +754,9 @@ void PageSpace::PrintHeapMapToJSONStream(Isolate* isolate,
     // "pages" is an array [page0, page1, ..., pageN], each page of the form
     // {"object_start": "0x...", "objects": [size, class id, size, ...]}
     // TODO(19445): Use ExclusivePageIterator once HeapMap supports large pages.
+    HeapIterationScope iteration(Thread::Current());
     MutexLocker ml(pages_lock_);
     MakeIterable();
-    NoSafepointScope no_safepoint;
     JSONArray all_pages(&heap_map, "pages");
     for (HeapPage* page = pages_; page != NULL; page = page->next()) {
       JSONObject page_container(&all_pages);