Revert "Remove Expect from core library."

tests/corelib/string_replace_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 StringReplaceTest {
   static testMain() {
     Expect.equals(
         "AtoBtoCDtoE", "AfromBtoCDtoE".replaceFirst("from", "to"));
 
     // Test with the replaced string at the begining.
     Expect.equals(
         "toABtoCDtoE", "fromABtoCDtoE".replaceFirst("from", "to"));
 
     // Test with the replaced string at the end.
@@ skipping 24 matching lines @@
     Expect.equals("to", "".replaceFirst("", "to"));
 
     // Test replacing the empty string.
     Expect.equals("toAtoBtoCto", "AtoBtoCto".replaceFirst("", "to"));
   }
 }
 
 main() {
   StringReplaceTest.testMain();
 }