Fix crash that happens when we send an object whose type is in a defer

tests/isolate/deferred_loaded_lib.dart

Index: tests/isolate/deferred_loaded_lib.dart
===================================================================
--- tests/isolate/deferred_loaded_lib.dart	(revision 0)
+++ tests/isolate/deferred_loaded_lib.dart	(working copy)
@@ -0,0 +1,9 @@
+// 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.
+
+class FromChildIsolate {
+  String toString() => 'from child isolate';
+  int get fld => 10;
+}
+