2nd attempt at adding streamListen/streamCancel to the service protocol.

runtime/vm/object.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.
 
 #include "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 13013 matching lines @@
   AddCommonObjectProperties(&jsobj, "Context", ref);
   jsobj.AddServiceId(*this);
 
   jsobj.AddProperty("length", num_variables());
 
   if (ref) {
     return;
   }
 
   const Context& parent_context = Context::Handle(parent());
-  jsobj.AddProperty("parent", parent_context);
+  if (!parent_context.IsNull()) {
+    jsobj.AddProperty("parent", parent_context);
+  }
 
   JSONArray jsarr(&jsobj, "variables");
   Object& var = Object::Handle();
   for (intptr_t index = 0; index < num_variables(); index++) {
     var = At(index);
     JSONObject jselement(&jsarr);
     jselement.AddProperty("value", var);
   }
 }
 
@@ skipping 7763 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart