Revert "Remove Expect from core library."

tests/language/switch6_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.
 // The break is in the right scope, http://b/3428700 was agreed upon.
 
-import "package:expect/expect.dart";
-
 class Switch6Test {
 
   static testMain() {
     var a = 0;
     var x = -1;
     switch (a) {
       case 0: {
         x = 0;
         break;
       }
       case 1:
         x = 1;
         break;
     }
     Expect.equals(0, x);
   }
 
 }
 
 main() {
   Switch6Test.testMain();
 }