Verify/document assumption about stack resource addresses.

runtime/vm/allocation.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_ALLOCATION_H_
 #define VM_ALLOCATION_H_
 
 #include "platform/assert.h"
 #include "vm/base_isolate.h"
 #include "vm/globals.h"
@@ skipping 47 matching lines @@
       BaseIsolate::AssertCurrent(isolate_);
     }
 #endif
   }
 
   // We can only create StackResources with Isolates, so provide the original
   // isolate to the subclasses. The only reason we have a BaseIsolate in the
   // 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_;
 
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(StackResource);
 };
 
 
 // Static allocated classes only contain static members and can never
@@ skipping 33 matching lines @@
   // deallocated by invoking DeleteAll() on the zone they live in.
   void operator delete(void* pointer) { UNREACHABLE(); }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ZoneAllocated);
 };
 
 }  // namespace dart
 
 #endif  // VM_ALLOCATION_H_