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

runtime/vm/service_test.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.
 
 // TODO(zra): Remove when tests are ready to enable.
 #include "platform/globals.h"
 #if !defined(TARGET_ARCH_ARM64)
 
 #include "include/dart_debugger_api.h"
 #include "vm/dart_api_impl.h"
@@ skipping 944 matching lines @@
                 address);
     EXPECT_SUBSTRING(buffer, handler.msg());
   }
 
   // Request native code at address. Expect the null code object back.
   address = last;
   service_msg = EvalF(h_lib, "[port, ['code', 'native-%" Px "'], [], []]",
                       address);
   Service::HandleIsolateMessage(isolate, service_msg);
   handler.HandleNextMessage();
-  EXPECT_STREQ("{\"type\":\"null\"}", handler.msg());
+  EXPECT_STREQ("{\"type\":\"Null\",\"id\":\"objects\\/null\","
+               "\"valueAsString\":\"null\"}",
+               handler.msg());
 
   // Request malformed native code.
   service_msg = EvalF(h_lib, "[port, ['code', 'native%" Px "'], [], []]",
                       address);
   Service::HandleIsolateMessage(isolate, service_msg);
   handler.HandleNextMessage();
   EXPECT_SUBSTRING("\"message\":\"Malformed code id:", handler.msg());
 }
 
 
@@ skipping 230 matching lines @@
     service_msg = Eval(lib, buf);
     Service::HandleIsolateMessage(isolate, service_msg);
     handler.HandleNextMessage();
     EXPECT_SUBSTRING("\"type\":\"@String\"", handler.msg());
     EXPECT_SUBSTRING("foobar", handler.msg());
   }
   // Expect null when no object is found.
   service_msg = Eval(lib, "[port, ['address', '7'], [], []]");
   Service::HandleIsolateMessage(isolate, service_msg);
   handler.HandleNextMessage();
-  EXPECT_SUBSTRING("\"type\":\"null\"", handler.msg());
+  // TODO(turnidge): Should this be a ServiceException instead?
+  EXPECT_STREQ("{\"type\":\"@Null\",\"id\":\"objects\\/null\","
+               "\"valueAsString\":\"null\"}",
+               handler.msg());
 }
 
 
 static const char* alpha_callback(
     const char* name,
     const char** arguments,
     intptr_t num_arguments,
     const char** option_keys,
     const char** option_values,
     intptr_t num_options,
@@ skipping 131 matching lines @@
   service_msg = Eval(h_lib, "[port, ['profile'], ['tags'], ['hidden']]");
   Service::HandleIsolateMessage(isolate, service_msg);
   handler.HandleNextMessage();
   // Expect error.
   EXPECT_SUBSTRING("\"type\":\"Error\"", handler.msg());
 }
 
 }  // namespace dart
 
 #endif  // !defined(TARGET_ARCH_ARM64)