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

runtime/vm/object_graph_test.cc

Index: runtime/vm/object_graph_test.cc
diff --git a/runtime/vm/object_graph_test.cc b/runtime/vm/object_graph_test.cc
index fb2329089956e7f544d044ac06dae58305bb94a2..becea26a88442af0fb00472b1b208b5b6374faf4 100644
--- a/runtime/vm/object_graph_test.cc
+++ b/runtime/vm/object_graph_test.cc
@@ -57,8 +57,6 @@ ISOLATE_UNIT_TEST_CASE(ObjectGraph) {
   intptr_t c_size = c.raw()->Size();
   intptr_t d_size = d.raw()->Size();
   {
-    // No more allocation; raw pointers ahead.
-    NoSafepointScope no_safepoint_scope;
     RawObject* b_raw = b.raw();

[danunez, 2017/08/05 17:24:11]

I think we can remove the NoSafepoint here because our mutator thread is not
going to be allocating and we shouldn't have a background thread doing any real
work in this test.

Am I correct in that?

If there is a background thread doing allocation in this test, we could see this
test come up later when background threads start allocating alongside mutators.

[rmacnak, 2017/08/07 22:37:51]

Nevermind, this is unsafe in the unlikely case that the test harness has the
background compiler running. Replaced with a SafepointOperationScope.

     // Clear handles to cut unintended retained paths.
     b = Array::null();