Make VM service id handling JSON-RPC 2 compliant.

runtime/vm/json_stream.h

Index: runtime/vm/json_stream.h
diff --git a/runtime/vm/json_stream.h b/runtime/vm/json_stream.h
index d314dbf62eae1cafa5bb861be4d802132fafca61..e581560d75932cbf2a0daf7f70c99570d1ab1c32 100644
--- a/runtime/vm/json_stream.h
+++ b/runtime/vm/json_stream.h
@@ -57,7 +57,7 @@ class JSONStream : ValueObject {
 
   void Setup(Zone* zone,
              Dart_Port reply_port,
-             const String& seq,
+             const Instance& seq,
              const String& method,
              const Array& param_keys,
              const Array& param_values);
@@ -100,7 +100,6 @@ class JSONStream : ValueObject {
   // otherwise.
   bool ParamIs(const char* key, const char* value) const;
 
-  const char* seq() const { return seq_; }
   const char* method() const { return method_; }
   const char** param_keys() const { return param_keys_; }
   const char** param_values() const { return param_values_; }
@@ -171,7 +170,7 @@ class JSONStream : ValueObject {
   RingServiceIdZone default_id_zone_;
   ServiceIdZone* id_zone_;
   Dart_Port reply_port_;
-  const char* seq_;
+  Instance& seq_;
   const char* method_;
   const char** param_keys_;
   const char** param_values_;