Fix for issue 21398 (only send "literal like" objects across isolates spawned using spawnURI

tests/isolate/issue_21398_child_isolate.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:isolate';
+import "package:expect/expect.dart";
+
+class FromChildIsolate {
+  String toString() => 'from child isolate';
+}
+
+main(List<String> args, message) {
+  var sendPort = message;
+  try {
+    sendPort.send(new FromChildIsolate());
+  } catch(error) {
+    Expect.isTrue(error is ArgumentError);
+    sendPort.send("Invalid Argument(s).");
+  }
+}