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

tests/isolate/issue_21398_child_isolate11.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 receivePort = new ReceivePort();
+  var sendPort = message;
+  sendPort.send(receivePort.sendPort);
+  receivePort.listen(
+    (msg) {
+      Expect.isTrue(msg is SendPort);
+      try {
+        msg.send(new FromChildIsolate());
+      } catch(error) {
+        Expect.isTrue(error is ArgumentError);
+        msg.send("Invalid Argument(s).");
+      }
+    },
+    onError: (e) => print('$e')
+  );
+}