Allow package: imports inside isolates

tests/standalone/package/sibling_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 sibling_isolate;
+
+import 'package:shared.dart'as shared;
+import 'dart:isolate';
+
+// This file is spawned from package_isolate_test.dart
+main() {
+  shared.output = 'isolate';
+  port.receive((msg, replyTo) {
+    replyTo.send(shared.output);
+  });
+}