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

runtime/vm/object_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"
 
 #include "vm/assembler.h"
 #include "vm/bigint_operations.h"
 #include "vm/class_finalizer.h"
@@ skipping 3996 matching lines @@
       return;
     }
     Object& handle = Object::Handle(obj);
     // Skip some common simple objects to run in reasonable time.
     if (handle.IsString() ||
         handle.IsArray() ||
         handle.IsLiteralToken()) {
       return;
     }
     JSONStream js;
-    handle.PrintToJSONStream(&js, false);
+    handle.PrintJSON(&js, false);
     EXPECT_SUBSTRING("\"type\":", js.ToCString());
   }
 };
 
 
-TEST_CASE(PrintToJSONStream) {
+TEST_CASE(PrintJSON) {
   Heap* heap = Isolate::Current()->heap();
   heap->CollectAllGarbage();
   JSONTypeVerifier verifier;
   heap->IterateObjects(&verifier);
 }
 
 #endif  // !defined(TARGET_ARCH_ARM64)
 
 }  // namespace dart