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

tests/isolate/src/APIv2_spawnBrowserTest.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("../../../client/testing/unittest/unittest.dart");
 #import("dart:dom"); // import added so test.dart can treat this as a webtest.
+#import('dart:isolate');
 
 child(ReceivePort port) {
   port.receive((msg, reply) => reply.send("re: $msg"));
 }
 
 main() {
   asyncTest("message - reply chain", 1, () {
     ReceivePort port = new ReceivePort();
     port.receive((msg, _) {
       expect(msg).equals("re: hi");
       port.close();
       callbackDone();
     });
 
     Isolate2 c = new Isolate2.fromCode(child);
     c.sendPort.send("hi", port.toSendPort());
   });
 }