Verify/document assumption about stack resource addresses.

runtime/vm/longjump.cc

Index: runtime/vm/longjump.cc
===================================================================
--- runtime/vm/longjump.cc	(revision 43776)
+++ runtime/vm/longjump.cc	(working copy)
@@ -55,11 +55,7 @@
   isolate->object_store()->set_sticky_error(error);
 
   // Destruct all the active StackResource objects.
-  StackResource* current_resource = isolate->top_resource();
-  while (current_resource != top_) {
-    current_resource->~StackResource();
-    current_resource = isolate->top_resource();
-  }
+  StackResource::Unwind(isolate, top_);
   longjmp(environment_, value);
   UNREACHABLE();
 }