Revert "Remove Expect from core library."

tests/language/nested_comments_test.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 testing nested comments
 
-import "package:expect/expect.dart";
-
 // /* nested comment */
 /*// nested comment */
 /*/* nested comment */*/
 /* entire function
 void main() {
   /* nested comment */
   Expect.isTrue(false); // nested
 }
 */
 
 /**
  test documentation
  /* nested comment */
 */
 main() {
   Expect.isTrue(true);
 }