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

tests/isolate/src/APIv2_spawnUriBrowserTest.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 URI
 #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');
 
 main() {
   asyncTest("isolate fromUri - send and reply ", 1, () {
     ReceivePort port = new ReceivePort();
     port.receive((msg, _) {
       expect(msg).equals("re: hi");
       port.close();
       callbackDone();
     });
 
     // TODO(eub): make this work for non-JS targets.
     Isolate2 c = new Isolate2.fromUri("./APIv2_spawnUriChildIsolate.js");
     c.sendPort.send("hi", port.toSendPort());
   });
 }