Revert "Remove Expect from core library."

tests/language/empty_block_case_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.
 
-import "package:expect/expect.dart";
-
 // Test that a case with an empty block does not fall through.
 
 class EmptyBlockCaseTest {
 
   static testMain() {
     var exception = null;
     try {
       switch (1) {
         case 1: {}
         case 2: Expect.equals(true, false);
       }
     } on FallThroughError catch (e) {
       exception = e;
     }
     Expect.equals(true, exception != null);
   }
 }
 
 main() {
   EmptyBlockCaseTest.testMain();
 }