Fix dart warnings.

runtime/observatory/bin/shell.dart

 // Copyright (c) 2014, 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.
 
 library shell;
 
 import 'package:observatory/service_io.dart';
 
 import 'dart:io';
 
 // Simple demo for service_io library. Connects to localhost on the default
 // port, picks the first isolate, reads requests from stdin, and prints
 // results to stdout. Example session:
 // <<< isolate isolates/1071334835
 // >>> /classes/40
 // <<< {"type":"Class","id":"classes\/40","name":"num","user_name":"num",...
 // >>> /objects/0
 // >>> {"type":"Array","class":{"type":"@Class","id":"classes\/62",...
 
 void repl(VM vm, Isolate isolate, String lastResult) {
   print(lastResult);
   Map params = {
     'objectId': stdin.readLineSync(),
   };
-  isolate.invokeRpc('getObject', params).then((String result) {
-    repl(vm, prefix, result);
+  isolate.invokeRpcNoUpgrade('getObject', params).then((Map result) {
+    repl(vm, isolate, result.toString());
   });
 }
 
 void main() {
   String addr = 'ws://localhost:8181/ws';
   new WebSocketVM(new WebSocketVMTarget(addr)).load().then((VM vm) {
     Isolate isolate = vm.isolates.first;
     repl(vm, isolate, 'isolate ${isolate.id}');
   });
 }