Make the PackageRoot test file option relative to the test file.

dart/tests/standalone/package/package_isolate_test.dart

 // Copyright (c) 2012, 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.
 
-// PackageRoot=tests/standalone/package/packages/
+// PackageRoot=packages/
 
 library package_isolate_test;
 import 'package:shared.dart' as shared;
 import 'dart:isolate';
 import '../../../pkg/unittest/lib/unittest.dart';
 
 expectResponse() {
   port.receive(expectAsync2((msg, r) {
     expect('isolate', msg);
     expect('main', shared.output);
@@ skipping 23 matching lines @@
     sendPort.send('sendPort', port.toSendPort());
   });
 }
 
 void isolate_main() {
   shared.output = 'isolate';
   port.receive((msg, replyTo) {
     replyTo.send(shared.output);
   });
 }