Implement spawnFunction from the new isolate api.

lib/isolate/isolate_api.dart

Index: lib/isolate/isolate_api.dart
===================================================================
--- lib/isolate/isolate_api.dart	(revision 5367)
+++ lib/isolate/isolate_api.dart	(working copy)
@@ -2,6 +2,12 @@
 // 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 IsolateSpawnException implements Exception {

[Siggi Cherem (dart-lang), 2012/03/13 23:00:14]

(nit): move this to the bottom, so that the top of this file contains the common
APIs that people will use. 
also + a 1-line comment so that it gets documented for api.dartlang.com

+  const IsolateSpawnException(String this._s);
+  String toString() => "IsolateSpawnException: '$_s'";
+  final String _s;
+}
+
 /**
  * The initial [ReceivePort] available by default for this isolate. This
  * [ReceivePort] is created automatically and it is commonly used to establish