Update unittest to new package layout.

tests/isolate/spawn_function_negative_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.
 
 // Negative test to make sure that we are reaching all assertions.
 #library('spawn_tests');
 #import('dart:isolate');
-#import('../../pkg/unittest/unittest.dart');
+#import('../../pkg/unittest/lib/unittest.dart');
 
 child() {
   port.receive((msg, reply) => reply.send('re: $msg'));
 }
 
 main() {
   test('message - reply chain', () {
     ReceivePort port = new ReceivePort();
     port.receive(expectAsync2((msg, _) {
       port.close();
       expect(msg, equals('re: hello')); // should be hi, not hello
     }));
 
     SendPort s = spawnFunction(child);
     s.send('hi', port.toSendPort());
   });
 }