Revert "Remove Expect from core library."

tests/language/label_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 to test check that we can parse labels.
 
-import "package:expect/expect.dart";
-
 
 class Helper {
 
   static int ticks;
 
   // Helper function to prevent endless loops in case labels or
   // break/continue is broken.
   static doAgain() {
     ++ticks;
     if (ticks > 300) {
@@ skipping 291 matching lines @@
     Expect.equals(111, Helper.test9());
     Expect.equals(111, Helper.test10());
     Expect.equals(111, Helper.test11());
     Expect.equals(111, Helper.test12());
   }
 }
 
 main() {
   LabelTest.testMain();
 }