Use VM type cast and save handles.

runtime/vm/isolate.cc

 // Copyright (c) 2012, 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.
 
 #include "vm/isolate.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "lib/mirrors.h"
 #include "vm/compiler_stats.h"
@@ skipping 91 matching lines @@
   if (message->IsOOB()) {
     // For now the only OOB messages are Mirrors messages.
     HandleMirrorsMessage(isolate_, message->reply_port(), msg);
     delete message;
   } else {
     const Object& result = Object::Handle(
         DartLibraryCalls::HandleMessage(
             message->dest_port(), message->reply_port(), msg));
     delete message;
     if (result.IsError()) {
-      Error& error = Error::Handle();
-      error ^= result.raw();
-      isolate_->object_store()->set_sticky_error(error);
+      isolate_->object_store()->set_sticky_error(Error::Cast(result));
       return false;
     }
     ASSERT(result.IsNull());
   }
   return true;
 }
 
 
 #if defined(DEBUG)
 void IsolateMessageHandler::CheckAccess() {
@@ skipping 321 matching lines @@
 
 
 void Isolate::VisitWeakPersistentHandles(HandleVisitor* visitor,
                                          bool visit_prologue_weak_handles) {
   if (api_state() != NULL) {
     api_state()->VisitWeakHandles(visitor, visit_prologue_weak_handles);
   }
 }
 
 }  // namespace dart