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

runtime/vm/service_test.cc

Index: runtime/vm/service_test.cc
diff --git a/runtime/vm/service_test.cc b/runtime/vm/service_test.cc
index cb5eb23c4b4d701fd2eb39c5a8f881bbaabb0b42..c4affeea3028af18e1c3ff94a36ef6eb43a35067 100644
--- a/runtime/vm/service_test.cc
+++ b/runtime/vm/service_test.cc
@@ -962,7 +962,9 @@ TEST_CASE(Service_Code) {
                       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 "'], [], []]",
@@ -1213,7 +1215,10 @@ TEST_CASE(Service_Address) {
   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());
 }