Revert "Remove Expect from core library."

tests/language/continue_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 for continue in for, do/while and while loops.
 
-import "package:expect/expect.dart";
-
 class ContinueTest {
   static testMain() {
     int i;
     int forCounter = 0;
     for (i = 0; i < 10; i++) {
       if (i > 3) continue;
       forCounter++;
     }
     Expect.equals(4, forCounter);
     Expect.equals(10, i);
@@ skipping 48 matching lines @@
       }
       i = 30;
     } while (false);
     Expect.equals(22, i);
   }
 }
 
 main() {
   ContinueTest.testMain();
 }