Move unittest from lib to pkg.

tests/isolate/isolate_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.
 
 // Dart test program for testing that isolates are spawned.
 
 #library('IsolateNegativeTest');
 #import('dart:isolate');
-#import('../../lib/unittest/unittest.dart');
+#import('../../pkg/unittest/unittest.dart');
 
 void entry() {
   port.receive((ignored, replyTo) {
     replyTo.send("foo", null);
   });
 }
 
 main() {
   test("ensure isolate code is executed", () {
     SendPort port = spawnFunction(entry);
     port.call("foo").then(expectAsync1((message) {
       Expect.equals(true, "Expected fail");   // <=-------- Should fail here.
     }));
   });
 }