Revert "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);
-}
-