Per isolate package root.

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';
+
+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 (Uri.parse(Platform.packageRoot) != SPAWN_PACKAGE_ROOT) {
+    throw "Wrong package root";
+  }
+  args[0].send(null);
+}
+