Verify/document assumption about stack resource addresses.

runtime/vm/allocation.cc

Index: runtime/vm/allocation.cc
===================================================================
--- runtime/vm/allocation.cc	(revision 43776)
+++ runtime/vm/allocation.cc	(working copy)
@@ -35,4 +35,20 @@
   return Allocate(size, zone);
 }
 
+
+void StackResource::Unwind(Isolate* isolate, uword stack_pointer) {
+  while (isolate->top_resource() != NULL &&
+         (reinterpret_cast<uword>(isolate->top_resource()) < stack_pointer)) {
+    isolate->top_resource()->~StackResource();
+  }
+#if defined(DEBUG)
+  // All remaining stack resources should be below stack_pointer.
+  StackResource* current = isolate->top_resource();
+  while (current != NULL) {
+    ASSERT(reinterpret_cast<uword>(current) > stack_pointer);
+    current = current->previous_;
+  }
+#endif  // DEBUG
+}
+
 }  // namespace dart