Fix checked mode failures in vmservice tests

tests/standalone/vmservice/isolate_code_test.dart

Index: tests/standalone/vmservice/isolate_code_test.dart
diff --git a/tests/standalone/vmservice/isolate_code_test.dart b/tests/standalone/vmservice/isolate_code_test.dart
index 4d680767873dcfc74548ff844c95dba62d8606d3..fea246ac218e8fe86bb55daa0188e5ec9ac6c583 100644
--- a/tests/standalone/vmservice/isolate_code_test.dart
+++ b/tests/standalone/vmservice/isolate_code_test.dart
@@ -34,8 +34,8 @@ class StackTraceTest extends VmServiceRequestHelper {
   StackTraceTest(port, id) :
       super('http://127.0.0.1:$port/$id/stacktrace');
 
-  int _aId;
-  int _cId;
+  String _aId;
+  String _cId;
   onRequestCompleted(Map reply) {
     Expect.equals('StackTrace', reply['type']);
     List members = reply['members'];
@@ -49,7 +49,7 @@ class StackTraceTest extends VmServiceRequestHelper {
 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(2);