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

runtime/vm/object.h

 // 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.
 
 #ifndef VM_OBJECT_H_
 #define VM_OBJECT_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/utils.h"
@@ skipping 116 matching lines @@
     ASSERT(Object::Handle(raw).Is##object());                                  \
     return reinterpret_cast<Raw##object*>(raw);                                \
   }                                                                            \
   static Raw##object* null() {                                                 \
     return reinterpret_cast<Raw##object*>(Object::null());                     \
   }                                                                            \
   virtual const char* ToCString() const;                                       \
   /* Object is printed as JSON into stream. If ref is true only a header */    \
   /* with an object id is printed. If ref is false the object is fully   */    \
   /* printed.                                                            */    \
-  virtual void PrintToJSONStream(JSONStream* stream, bool ref = true) const;   \
   virtual const char* JSONType(bool ref) const {                               \
     return ref ? "@"#object : ""#object;                                       \
   }                                                                            \
   static const ClassId kClassId = k##object##Cid;                              \
+ protected:  /* NOLINT */                                                      \
+  virtual void PrintJSONImpl(JSONStream* stream, bool ref) const;              \
  private:  /* NOLINT */                                                        \
   /* Initialize the handle based on the raw_ptr in the presence of null. */    \
   static void initializeHandle(object* obj, RawObject* raw_ptr) {              \
     if (raw_ptr != Object::null()) {                                           \
       obj->SetRaw(raw_ptr);                                                    \
     } else {                                                                   \
       obj->raw_ = Object::null();                                              \
       object fake_object;                                                      \
       obj->set_vtable(fake_object.vtable());                                   \
     }                                                                          \
@@ skipping 109 matching lines @@
   bool IsNull() const { return raw_ == null_; }
 
   virtual const char* ToCString() const {
     if (IsNull()) {
       return "null";
     } else {
       return "Object";
     }
   }
 
-  virtual void PrintToJSONStream(JSONStream* stream, bool ref = true) const {
-    JSONObject jsobj(stream);
-    jsobj.AddProperty("type", JSONType(ref));
-  }
+  void PrintJSON(JSONStream* stream, bool ref = true) const;
 
   virtual const char* JSONType(bool ref) const {
     return IsNull() ? "null" : "Object";
   }
 
   // Returns the name that is used to identify an object in the
   // namespace dictionary.
   // Object::DictionaryName() returns String::null(). Only subclasses
   // of Object that need to be entered in the library and library prefix
   // namespaces need to provide an implementation.
@@ skipping 252 matching lines @@
     if (!value->IsHeapObject()) return;
     if (value->IsNewObject() && raw()->IsOldObject() &&
         !raw()->IsRemembered()) {
       raw()->SetRememberedBit();
       Isolate::Current()->store_buffer()->AddObject(raw());
     }
   }
 
   RawObject* raw_;  // The raw object reference.
 
+ protected:
+  virtual void PrintJSONImpl(JSONStream* stream, bool ref) const {
+    JSONObject jsobj(stream);
+    jsobj.AddProperty("type", JSONType(ref));
+  }
+
  private:
   static intptr_t NextFieldOffset() {
     // Indicates this class cannot be extended by dart code.
     return -kWordSize;
   }
 
   static void InitializeObject(uword address, intptr_t id, intptr_t size);
 
   static void RegisterClass(const Class& cls,
                             const String& name,
@@ skipping 3609 matching lines @@
   bool IsValidFieldOffset(intptr_t offset) const;
 
   static intptr_t NextFieldOffset() {
     return sizeof(RawInstance);
   }
 
   // TODO(iposva): Determine if this gets in the way of Smi.
   HEAP_OBJECT_IMPLEMENTATION(Instance, Object);
   friend class Class;
   friend class Closure;
+  friend class DeferredObject;
   friend class SnapshotWriter;
   friend class StubCode;
   friend class TypedDataView;
-  friend class DeferredObject;
 };
 
 
 class LibraryPrefix : public Instance {
  public:
   RawString* name() const { return raw_ptr()->name_; }
   virtual RawString* DictionaryName() const { return name(); }
 
   RawArray* imports() const { return raw_ptr()->imports_; }
   intptr_t num_imports() const { return raw_ptr()->num_imports_; }
@@ skipping 2782 matching lines @@
 
 
 RawObject* MegamorphicCache::GetTargetFunction(const Array& array,
                                                intptr_t index) {
   return array.At((index * kEntryLength) + kTargetFunctionIndex);
 }
 
 }  // namespace dart
 
 #endif  // VM_OBJECT_H_