Allow package: imports inside isolates

tests/standalone/package/test_folder/folder_isolate.dart

+// Copyright (c) 2012, 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.
+
+library folder_isolate;
+
+// this is a package that's not available to the main isolate
+import 'package:folder_lib.dart' as isolate_package;
+import 'dart:isolate';
+
+// This file is spawned from package_isolate_test.dart
+main() {
+  isolate_package.count = 1;
+  port.receive((msg, replyTo) {
+    replyTo.send('isolate');
+  });
+}