Revert "Remove Expect from core library."

tests/language/const_escape_frog_test.dart

Index: tests/language/const_escape_frog_test.dart
diff --git a/tests/language/const_escape_frog_test.dart b/tests/language/const_escape_frog_test.dart
index ea39cc368a06ff1127f55ba26f65d47156894f9a..776ea8e2b2544b126f4d999997c33a16b7cb1cf0 100644
--- a/tests/language/const_escape_frog_test.dart
+++ b/tests/language/const_escape_frog_test.dart
@@ -1,11 +1,3 @@
-// Copyright (c) 2013, 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 division by power of two.
-// Test that results before and after optimization are the same.
-
-import "package:expect/expect.dart";
-
 class Foo {
   const Bar<Foo> bar = const Bar/* comment here use to trigger bug 323 */();
 }