Add package config support to dart:isolate

tests/isolate/package_config_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_CONFIG = "foobar:///no/such/file/";
+
+void main([args, port]) async {
+  if (port != null) {
+    testPackageConfig(port);
+    return;
+  }
+  var p = new RawReceivePort();
+  Isolate.spawnUri(Platform.script,
+                   [],
+                   p.sendPort,
+                   packageConfig: Uri.parse(SPAWN_PACKAGE_CONFIG));
+  p.handler = (msg) {
+    p.close();
+    if (msg[0] != SPAWN_PACKAGE_CONFIG) {
+      throw "Bad package config in child isolate: ${msg[0]}";
+    }
+    if (msg[1] != null) {
+      throw "Non-null loaded package config in isolate: ${msg[1]}";
+    }
+    print("SUCCESS");
+  };
+  print("Spawning isolate's package config: ${await Isolate.packageConfig}");
+}
+
+testPackageConfig(port) async {
+  var packageConfigStr = Platform.packageConfig;
+  var packageConfig = await Isolate.packageConfig;
+  print("Spawned isolate's package config flag: $packageConfigStr");
+  print("Spawned isolate's loaded package config: $packageConfig");
+  port.send([packageConfigStr, packageConfig?.toString()]);
+}