Fix test in checked mode.

tests/isolate/package_root_test.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.
 
 import 'dart:io';
 import 'dart:isolate';
 
-const SPAWN_PACKAGE_ROOT = "otherPackageRoot";
+final SPAWN_PACKAGE_ROOT = Uri.parse("otherPackageRoot");
 
 void main([args, port]) {
   if (port != null) {
     testPackageRoot(args);
     return;
   }
   var p = new ReceivePort();
   Isolate.spawnUri(Platform.script,
                    [p.sendPort, Platform.packageRoot],
                    {},
                    packageRoot: SPAWN_PACKAGE_ROOT);
   p.listen((msg) {
     p.close();
   });
 }
 
 
 void testPackageRoot(args) {
   var parentPackageRoot = args[1];
   if (parentPackageRoot == Platform.packageRoot) {
     throw "Got parent package root";
   }
-  if (Platform.packageRoot != SPAWN_PACKAGE_ROOT) {
+  if (Uri.parse(Platform.packageRoot) != SPAWN_PACKAGE_ROOT) {
     throw "Wrong package root";
   }
   args[0].send(null);
 }