Assert no VM handles are created in the zone that belongs to a ApiNativeScopes.

runtime/vm/handles.h

 // 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.
 
 #ifndef VM_HANDLES_H_
 #define VM_HANDLES_H_
 
 #include "vm/allocation.h"
 #include "vm/flags.h"
 #include "vm/os.h"
@@ skipping 265 matching lines @@
   // Allocates a handle in 'zone', which must be the current zone. This handle
   // will be destroyed when the current zone is destroyed.
   static uword AllocateZoneHandle(Zone* zone);
 
   // Returns true if specified handle is a zone handle.
   static bool IsZoneHandle(uword handle);
 
   // Returns number of handles, these functions are used for testing purposes.
   static int ScopedHandleCount();
   static int ZoneHandleCount();
+
+  friend class ApiZone;
+  friend class ApiNativeScope;
 };
 
 
 // The class HandleScope is used to start a new handles scope in the code.
 // It is used as follows:
 // {
 //   HANDLESCOPE(isolate);
 //   ....
 //   .....
 //   code that creates some scoped handles.
@@ skipping 52 matching lines @@
 };
 #endif  // defined(DEBUG)
 
 // Macro to start a no handles scope in the code.
 #define NOHANDLESCOPE(isolate)                                                 \
     dart::NoHandleScope no_vm_internal_handles_scope_(isolate);
 
 }  // namespace dart
 
 #endif  // VM_HANDLES_H_