Verify/document assumption about stack resource addresses.

runtime/vm/allocation.h

Index: runtime/vm/allocation.h
===================================================================
--- runtime/vm/allocation.h	(revision 43776)
+++ runtime/vm/allocation.h	(working copy)
@@ -65,6 +65,10 @@
   // StackResource is to break the header include cycles.
   Isolate* isolate() const { return reinterpret_cast<Isolate*>(isolate_); }
 
+#if defined(DEBUG)
+  StackResource* previous() const { return previous_; }
+#endif  // DEBUG
+
  private:
   BaseIsolate* const isolate_;  // Current isolate for this stack resource.
   StackResource* previous_;