Update unittest to new package layout.

tests/isolate/spawn_function_custom_class_test.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.
 
 // Create a user-defined class in a new isolate.
 //
 // Regression test for vm bug 2235: We were forgetting to finalize
 // classes in new isolates started using the v2 api.
 
 #library('spawn_tests');
 #import('dart:isolate');
-#import('../../pkg/unittest/unittest.dart');
+#import('../../pkg/unittest/lib/unittest.dart');
 
 class MyClass {
   var myVar = 'there';
   myFunc(msg) {
     return '$msg $myVar';
   }
 }
 
 child() {
   port.receive((msg, reply) {
     reply.send('re: ${new MyClass().myFunc(msg)}');
   });
 }
 
 main() {
   test('message - reply chain', () {
     ReceivePort port = new ReceivePort();
     port.receive(expectAsync(msg, _) {
       port.close();
       expect(msg, equals('re: hi there'));
     });
 
     SendPort s = spawnFunction(child);
     s.send('hi', port.toSendPort());
   });
 }