Set VMTag from stubs when transitioning between Dart and Native

runtime/vm/isolate.h

 // Copyright (c) 2013, 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.
 
 #ifndef VM_ISOLATE_H_
 #define VM_ISOLATE_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/thread.h"
@@ skipping 158 matching lines @@
   static intptr_t top_context_offset() {
     return OFFSET_OF(Isolate, top_context_);
   }
 
   uword top_exit_frame_info() const { return top_exit_frame_info_; }
   void set_top_exit_frame_info(uword value) { top_exit_frame_info_ = value; }
   static intptr_t top_exit_frame_info_offset() {
     return OFFSET_OF(Isolate, top_exit_frame_info_);
   }
 
+  static intptr_t vm_tag_offset() {
+    return OFFSET_OF(Isolate, vm_tag_);
+  }
+
   ApiState* api_state() const { return api_state_; }
   void set_api_state(ApiState* value) { api_state_ = value; }
 
   StubCode* stub_code() const { return stub_code_; }
   void set_stub_code(StubCode* value) { stub_code_ = value; }
 
   LongJumpScope* long_jump_base() const { return long_jump_base_; }
   void set_long_jump_base(LongJumpScope* value) { long_jump_base_ = value; }
 
   TimerList& timer_list() { return timer_list_; }
@@ skipping 469 matching lines @@
   char* script_url_;
   char* library_url_;
   char* class_name_;
   char* function_name_;
   char* exception_callback_name_;
 };
 
 }  // namespace dart
 
 #endif  // VM_ISOLATE_H_