Add Dart API scoping around the call to Dart_HandleMessage.

runtime/vm/isolate.cc

 // Copyright (c) 2011, 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 "vm/assert.h"
 #include "vm/bigint_store.h"
@@ skipping 230 matching lines @@
   ASSERT(post_message_callback() == &StandardPostMessageCallback);
   ASSERT(close_port_callback() == &StandardClosePortCallback);
 
   while (active_ports() > 0) {
     ASSERT(this == Isolate::Current());
     Zone zone(this);
     HandleScope handle_scope(this);
 
     PortMessage* message = message_queue()->Dequeue(0);
     if (message != NULL) {
+      Dart_EnterScope();
       Dart_Handle result = Dart_HandleMessage(
           message->dest_port(), message->reply_port(), message->data());
       if (Dart_IsError(result)) {
+        // TODO(turnidge): Consider passing this error out to
+        // Dart_RunLoop so that the embedder can choose how to handle
+        // it.
         fprintf(stderr, "%s\n", Dart_GetError(result));
+        Dart_ExitScope();
         exit(255);
       }
+      Dart_ExitScope();
       delete message;
     }
   }
 }
 
 
 void Isolate::VisitObjectPointers(ObjectPointerVisitor* visitor,
                                   bool validate_frames) {
   ASSERT(visitor != NULL);
 
@@ skipping 22 matching lines @@
   // Visit all objects in the code index table.
   if (code_index_table() != NULL) {
     code_index_table()->VisitObjectPointers(visitor);
   }
 
   // Visit the top context which is stored in the isolate.
   visitor->VisitPointer(reinterpret_cast<RawObject**>(&top_context_));
 }
 
 }  // namespace dart