Revert "Remove Expect from core library."

tests/language/switch_scope_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.
 // Test that a new scope is introduced for each switch case.
 
-import "package:expect/expect.dart";
-
 class SwitchScopeTest {
   static testMain() {
     switch(1) {
       case 1:
         final v = 1;
         break;
       case 2:
         final v = 2;
         Expect.equals(2, v);
         break;
       default:
         final v = 3;
         break;
     }
   }
 }
 
 main() {
   SwitchScopeTest.testMain();
 }