isolates refactor: this change introduces 'dart:isolate' as a library. This is a

tests/isolate/src/APIv2_spawnStandaloneTest.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.
 
 // example of spawning an isolate from a closure
 #library('spawn_tests');
+#import('dart:isolate');
 
 child(ReceivePort port) {
   port.receive((msg, reply) => reply.send("re: $msg"));
 }
 
 main() {
   ReceivePort port = new ReceivePort();
   port.receive((msg, _) {
     Expect.equals("re: hi", msg);
     port.close();
   });
 
   Isolate2 c = new Isolate2.fromCode(child);
   c.sendPort.send("hi", port.toSendPort());
 }