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

tests/isolate/src/APIv2_spawnUriChildIsolate.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.
 
 // Child isolate code to be spawned from a URI to this file.
+#library('SpawnUriChildIsolate');
+#import('dart:isolate');
 
 void isolateMain(ReceivePort port) {
   port.receive((msg, reply) => reply.send("re: $msg"));
 }
 
 // Just for frog's sake.  TODO(eub): clean this up when we're able to.
 main() {
   if (false) {
     isolateMain(null);
     new ReceivePort();
   }
 }