Added a test for issue 12474.

tests/isolate/browser/issue_12474_test.dart

+// 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.
+
+// Note: the following comment is used by test.dart to additionally compile the
+// other isolate's code.
+// OtherScripts=issue_12474_child.dart
+// OtherScripts=package/issue_12474_lib.dart
+
+import 'dart:isolate';
+
+final SPAWN_PACKAGE_ROOT = Uri.parse(".");
+
+void main([args, port]) {
+  var p = new ReceivePort();
+  Isolate.spawnUri(Uri.parse("issue_12474_child.dart"),
+                   [p.sendPort],
+                   {},
+                   packageRoot: SPAWN_PACKAGE_ROOT);
+  p.listen((msg) {
+    print("Received message");
+    p.close();
+  });
+}