Add package config support to dart:isolate

tests/isolate/package_root_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.
+
+import 'dart:io';
+import 'dart:isolate';
+
+final SPAWN_PACKAGE_ROOT = "file:///no/such/file/";
+
+void main([args, port]) async {
+  if (port != null) {
+    testPackageRoot(port);
+    return;
+  }
+  var p = new RawReceivePort();
+  Isolate.spawnUri(Platform.script,
+                   [],
+                   p.sendPort,
+                   packageRoot: Uri.parse(SPAWN_PACKAGE_ROOT));
+  p.handler = (msg) {
+    p.close();
+    if (msg != SPAWN_PACKAGE_ROOT) {
+      throw "Bad package root in child isolate: $msg";
+    }
+    print("SUCCESS");
+  };
+  print("Spawning isolate's package root: ${await Isolate.packageRoot}");
+}
+
+testPackageRoot(port) async {
+  var packageRoot = await Isolate.packageRoot;
+  print("Spawned isolate's package root: $packageRoot");
+  port.send(packageRoot.toString());
+}