Enable iterating over roots from helper threads by moving stack walking.

runtime/vm/thread_test.cc

Index: runtime/vm/thread_test.cc
diff --git a/runtime/vm/thread_test.cc b/runtime/vm/thread_test.cc
index 37d0c0ea04b85bb8fae19420b95cc9c321b0618a..9290d637d30f2e182325010f9ef3e7d2472ea619 100644
--- a/runtime/vm/thread_test.cc
+++ b/runtime/vm/thread_test.cc
@@ -133,8 +133,9 @@ class TaskWithZoneAllocation : public ThreadPool::Task {
       EXPECT(smi.Value() == unique_smi);
       ObjectCounter counter(isolate_, &smi);
       // Ensure that our particular zone is visited.
-      // TODO(koda): Remove "->thread_registry()" after updating stack walker.
-      isolate_->thread_registry()->VisitObjectPointers(&counter);
+      isolate_->IterateObjectPointers(&counter,
+                                      /* visit_prologue_weak_handles = */ true,
+                                      /* validate_frames = */ true);

[Ivan Posva, 2015/08/11 17:41:31]

ditto

       EXPECT_EQ(1, counter.count());
 
       char* unique_chars = zone->PrintToString("unique_str_%" Pd, id_);