VM Service: Include a response id when enumerating scripts.

runtime/vm/service.cc

Index: runtime/vm/service.cc
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index 045dbe45b8ace845c2ac78ac8302b153a966901c..5ca1df3257194e98d2653d3c7d30197438abd828 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -1252,6 +1252,7 @@ static bool HandleObjects(Isolate* isolate, JSONStream* js) {
 static bool HandleScriptsEnumerate(Isolate* isolate, JSONStream* js) {
   JSONObject jsobj(js);
   jsobj.AddProperty("type", "ScriptList");
+  jsobj.AddProperty("id", "scripts");
   JSONArray members(&jsobj, "members");
   const GrowableObjectArray& libs =
       GrowableObjectArray::Handle(isolate->object_store()->libraries());