Always use the same json for null in the vm service.

runtime/vm/json_test.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.
 
 // TODO(zra): Remove when tests are ready to enable.
 #include "platform/globals.h"
 #if !defined(TARGET_ARCH_ARM64)
 
 #include "platform/assert.h"
 #include "platform/json.h"
@@ skipping 284 matching lines @@
 
 
 TEST_CASE(JSON_JSONStream_DartObject) {
   JSONStream js;
   {
     JSONArray jsarr(&js);
     jsarr.AddValue(Object::Handle(Object::null()));
     JSONObject jsobj(&jsarr);
     jsobj.AddProperty("object_key", Object::Handle(Object::null()));
   }
-  EXPECT_STREQ("[{\"type\":\"null\"},{\"object_key\":{\"type\":\"null\"}}]",
+  EXPECT_STREQ("[{\"type\":\"@Null\",\"id\":\"objects\\/null\","
+               "\"valueAsString\":\"null\"},"
+               "{\"object_key\":{\"type\":\"@Null\",\"id\":\"objects\\/null\","
+               "\"valueAsString\":\"null\"}}]",
                js.ToCString());
 }
 
 TEST_CASE(JSON_JSONStream_EscapedString) {
   JSONStream js;
   {
     JSONArray jsarr(&js);
     jsarr.AddValue("Hel\"\"lo\r\n\t");
   }
   EXPECT_STREQ("[\"Hel\\\"\\\"lo\\r\\n\\t\"]", js.ToCString());
@@ skipping 17 matching lines @@
   EXPECT(!js.HasOption("lizard"));
   EXPECT(js.HasOption("dog"));
   EXPECT(js.HasOption("cat"));
   EXPECT(js.OptionIs("cat", "banana"));
   EXPECT(!js.OptionIs("dog", "banana"));
 }
 
 }  // namespace dart
 
 #endif  // !defined(TARGET_ARCH_ARM64)