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

tests/isolate/src/Mixed2Test.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 heavy and light isolates can be mixed.
 
 #library('Mixed2Test');
+#import("dart:isolate");
 #import('TestFramework.dart');
 
 // We want to send a message from the main-isolate to a chain of different
 // isolates and then get a reply back.
 // In the following description heavy2 is not used, because it is shut down
 // just after heaving created heavy2.light1,2 and light3.
 // main->
 //   heavy1->heavy1.light1->heavy1.light2->heavy1.light3->
 //   heavy2.light1->heavy2.light2->heavy2.light3->
 //   heavy3->heavy3.pong
@@ skipping 107 matching lines @@
           }));
         }));
       }));
     }));
   }));
 }
 
 main() {
   runTests([test]);
 }