Fix incorrect usage of NoGCScope in previous change (dart objects are being

runtime/vm/dart_api_impl.cc

 // 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.
 
 #include "include/dart_api.h"
 #include "include/dart_mirrors_api.h"
 #include "include/dart_native_api.h"
 
 #include "platform/assert.h"
 #include "vm/bigint_operations.h"
@@ skipping 3583 matching lines @@
 DART_EXPORT void Dart_SetReturnValue(Dart_NativeArguments args,
                                      Dart_Handle retval) {
   NativeArguments* arguments = reinterpret_cast<NativeArguments*>(args);
   Isolate* isolate = arguments->isolate();
   CHECK_ISOLATE(isolate);
   if ((retval != Api::Null()) && (!Api::IsInstance(retval))) {
     const Object& ret_obj = Object::Handle(Api::UnwrapHandle(retval));
     FATAL1("Return value check failed: saw '%s' expected a dart Instance.",
            ret_obj.ToCString());
   }
+  ASSERT(retval != 0);
   Api::SetReturnValue(arguments, retval);
 }
 
 
 DART_EXPORT void Dart_SetBooleanReturnValue(Dart_NativeArguments args,
                                             bool retval) {
   NativeArguments* arguments = reinterpret_cast<NativeArguments*>(args);
   arguments->SetReturn(retval ? Bool::True() : Bool::False());
 }
 
@@ skipping 494 matching lines @@
   }
   {
     NoGCScope no_gc;
     RawObject* raw_obj = obj.raw();
     isolate->heap()->SetPeer(raw_obj, peer);
   }
   return Api::Success();
 }
 
 }  // namespace dart