Add NoSafepointScope to ObjectPointerVisitor.

runtime/vm/allocation.h

Index: runtime/vm/allocation.h
diff --git a/runtime/vm/allocation.h b/runtime/vm/allocation.h
index 3947f0e79881db294753869df3f3185280f49837..74a2d7d6d17aa85c4dd05d34d3fd9231088e2475 100644
--- a/runtime/vm/allocation.h
+++ b/runtime/vm/allocation.h
@@ -134,6 +134,27 @@ class ZoneAllocated {
   DISALLOW_COPY_AND_ASSIGN(ZoneAllocated);
 };
 
+
+
+// Within a NoSafepointScope, the thread must not reach any safepoint. Used
+// around code that manipulates raw object pointers directly without handles.
+#if defined(DEBUG)
+class NoSafepointScope : public StackResource {
+ public:
+  NoSafepointScope();
+  ~NoSafepointScope();
+ private:
+  DISALLOW_COPY_AND_ASSIGN(NoSafepointScope);
+};
+#else  // defined(DEBUG)
+class NoSafepointScope : public ValueObject {
+ public:
+  NoSafepointScope() {}
+ private:
+  DISALLOW_COPY_AND_ASSIGN(NoSafepointScope);
+};
+#endif  // defined(DEBUG)
+
 }  // namespace dart
 
 #endif  // VM_ALLOCATION_H_