Do not JSON encode the 'result' of a service rpc.

runtime/vm/service/client.dart

Index: runtime/vm/service/client.dart
diff --git a/runtime/vm/service/client.dart b/runtime/vm/service/client.dart
index 0d0b253f0fa9de64cd2a23634590e0b2e65db152..56051f8deaf13c873ffb2e0bc2ac98767372f1cd 100644
--- a/runtime/vm/service/client.dart
+++ b/runtime/vm/service/client.dart
@@ -25,16 +25,16 @@ abstract class Client {
       // Send message to service.
       service.route(message).then((response) {
         // Call post when the response arrives.
-        post(seq, response);
+        post(response);
       });
     } catch (e, st) {
       message.setErrorResponse('Internal error: $e');
-      post(seq, message.response);
+      post(message.response);
     }
   }
 
   // Sends a result to the client.  Implemented in subclasses.
-  void post(var seq, dynamic result);
+  void post(dynamic result);
 
   dynamic toJson() {
     return {