Verify/document assumption about stack resource addresses.

runtime/vm/exceptions.cc

Index: runtime/vm/exceptions.cc
===================================================================
--- runtime/vm/exceptions.cc	(revision 43776)
+++ runtime/vm/exceptions.cc	(working copy)
@@ -281,12 +281,8 @@
 #else
   // Prepare for unwinding frames by destroying all the stack resources
   // in the previous frames.
+  StackResource::Unwind(isolate, stack_pointer);
 
-  while (isolate->top_resource() != NULL &&
-         (reinterpret_cast<uword>(isolate->top_resource()) < stack_pointer)) {
-    isolate->top_resource()->~StackResource();
-  }
-
   // Call a stub to set up the exception object in kExceptionObjectReg,
   // to set up the stacktrace object in kStackTraceObjectReg, and to
   // continue execution at the given pc in the given frame.