Fix checked mode failures in vmservice tests

tests/standalone/vmservice/isolate_library_test.dart

Index: tests/standalone/vmservice/isolate_library_test.dart
diff --git a/tests/standalone/vmservice/isolate_library_test.dart b/tests/standalone/vmservice/isolate_library_test.dart
index a42d8b1862ade11031a8c21b74016a41094a65bc..18d6caba35e1fa99c83a487cea4d9b26eabdbc6e 100644
--- a/tests/standalone/vmservice/isolate_library_test.dart
+++ b/tests/standalone/vmservice/isolate_library_test.dart
@@ -22,7 +22,7 @@ class RootLibraryTest extends VmServiceRequestHelper {
   RootLibraryTest(port, id) :
       super('http://127.0.0.1:$port/$id/library');
 
-  int _libId;
+  String _libId;
   onRequestCompleted(Map reply) {
     Expect.equals('Library', reply['type']);
     Expect.equals('isolate_stacktrace_command_script', reply['name']);
@@ -33,7 +33,7 @@ class RootLibraryTest 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);