Added a test for issue 12474.

tests/isolate/browser/issue_12474_child.dart

Index: tests/isolate/browser/issue_12474_child.dart
===================================================================
--- tests/isolate/browser/issue_12474_child.dart	(revision 0)
+++ tests/isolate/browser/issue_12474_child.dart	(working copy)
@@ -0,0 +1,10 @@
+// Copyright (c) 2015, 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:isolate';
+import 'package:issue_12474_lib.dart';
+
+void main([args, port]) {
+  testPackageRoot(args);
+}