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

tests/isolate/src/NestedSpawnTest.dart

 // Copyright (c) 2011, 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.
 
 // Dart test program for testing that isolates can spawn other isolates.
 
 #library('NestedSpawnTest');
+#import("dart:isolate");
 #import('TestFramework.dart');
 
 class IsolateA extends Isolate {
   IsolateA() : super.heavy();
 
   void main() {
     this.port.receive((msg, replyTo) {
       Expect.equals("launch nested!", msg);
       new IsolateB().spawn().then((SendPort p) {
         p.call("alive?").receive((msg, ignored) {
@@ skipping 23 matching lines @@
     port.call("launch nested!").receive(expect.runs2((msg, ignored) {
       Expect.equals(499, msg);
       expect.succeeded();
     }));
   });
 }
 
 main() {
   runTests([test]);
 }