Add package config support to dart:isolate

tests/isolate/scenarios/automatic_resolution_root/package_resolve_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 PACKAGE_URI = "package:foo/bar.dart";
+
+void main([args, port]) async {
+  if (port != null) {
+    testPackageResolution(port);
+    return;
+  }
+  var p = new RawReceivePort();
+  Isolate.spawnUri(Platform.script,
+                   [],
+                   p.sendPort,
+                   automaticPackageResolution: true);
+  p.handler = (msg) {
+    p.close();
+    if (msg is! List) {
+      print(msg.runtimeType);
+      throw "Failure return from spawned isolate:\n\n$msg";
+    }
+    var child_pkg_root = Platform.script.resolve("packages/");
+    if (msg[0] != child_pkg_root.toString()) {
+      throw "Bad package root in child isolate: ${msg[0]}.\n"
+            "Expected: $child_pkg_root";
+    }
+    var child_pkg_path = child_pkg_root.resolve("foo/bar.dart");
+    if (msg[1] != child_pkg_path.toString()) {
+      throw "Package path not matching: ${msg[1]}\n"
+            "Expected $child_pkg_path";
+    }
+    print("SUCCESS");
+  };
+  print("Spawning isolate's package root: ${await Isolate.packageRoot}");
+}
+
+testPackageResolution(port) async {
+  try {
+    var packageRootStr = Platform.packageRoot;
+    var packageConfigStr = Platform.packageConfig;
+    var packageRoot = await Isolate.packageRoot;
+    var resolvedPkg = await Isolate.resolvePackageUri(Uri.parse(PACKAGE_URI));
+    print("Spawned isolate's package root flag: $packageRootStr");
+    print("Spawned isolate's package config flag: $packageConfigStr");
+    print("Spawned isolate's loaded package root: $packageRoot");
+    print("Spawned isolate's resolved package path: $resolvedPkg");
+    port.send([packageRoot?.toString(), resolvedPkg?.toString()]);
+  } catch (e, s) {
+    port.send("$e\n$s\n");
+  }
+}