Revert "Remove Expect from core library."

tests/corelib/string_replace_all_test.dart

 // Copyright (c) 2012, 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";
-
 testReplaceAll() {
   Expect.equals(
       "aXXcaXXdae", "abcabdae".replaceAll("b", "XX"));
 
   // Test with the replaced string at the begining.
   Expect.equals(
       "XXbcXXbdXXe", "abcabdae".replaceAll("a", "XX"));
 
   // Test with the replaced string at the end.
   Expect.equals(
@@ skipping 123 matching lines @@
       "<>a<bc>a<bd>a<e>",
       "abcabdae".splitMapJoin("a", onNonMatch: wrap));
 
 }
 
 main() {
   testReplaceAll();
   testReplaceAllMapped();
   testSplitMapJoin();
 }