Fix checked mode failures in vmservice tests

tests/standalone/vmservice/isolate_echo_test.dart

 // Copyright (c) 2013, 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 isolate_echo_test;
 
 import 'test_helper.dart';
 import 'package:expect/expect.dart';
 
 class EchoRequestTest extends VmServiceRequestHelper {
@@ skipping 18 matching lines @@
     Expect.equals('b', reply['message']['option_values'][0]);
     Expect.equals('', reply['message']['option_values'][1]);
     Expect.equals('e', reply['message']['option_values'][2]);
     Expect.equals('w', reply['message']['option_values'][3]);
   }
 }
 
 class IsolateListTest extends VmServiceRequestHelper {
   IsolateListTest(port) : super('http://127.0.0.1:$port/isolates');
 
-  int _isolateId;
+  String _isolateId;
   onRequestCompleted(Map reply) {
     IsolateListTester tester = new IsolateListTester(reply);
     tester.checkIsolateCount(1);
     tester.checkIsolateNameContains('isolate_echo_script.dart');
     _isolateId = reply['members'][0]['id'];
   }
 }
 
 
 main() {
   var process = new TestLauncher('isolate_echo_script.dart');
   process.launch().then((port) {
     var test = new IsolateListTest(port);
     test.makeRequest().then((_) {
       var echoRequestTest = new EchoRequestTest(port, test._isolateId);
       echoRequestTest.makeRequest().then((_) {
         process.requestExit();
       });
     });
   });
 }