Revert "Remove Expect from core library."

tests/corelib/string_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";
-
 class StringCaseTest {
 
   static testMain() {
     testLowerUpper();
   }
 
   static testLowerUpper() {
     var a = "Stop! Smell the Roses.";
     var allLower = "stop! smell the roses.";
     var allUpper = "STOP! SMELL THE ROSES.";
     Expect.equals(allUpper, a.toUpperCase());
     Expect.equals(allLower, a.toLowerCase());
   }
 }
 
 main() {
   StringCaseTest.testMain();
 }