Revert "Remove Expect from core library."

tests/language/break_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 breaks in for, do/while and while loops.
 
-import "package:expect/expect.dart";
-
 class BreakTest {
   static testMain() {
     int i;
     int forCounter = 0;
     for (i = 0; i < 10; i++) {
       forCounter++;
       if (i > 3) break;
     }
     Expect.equals(5, forCounter);
     Expect.equals(4, i);
@@ skipping 26 matching lines @@
         break L;
       }
     }
     Expect.equals(6, i);
   }
 }
 
 main() {
   BreakTest.testMain();
 }