Revert "Remove Expect from core library."

tests/corelib/string_slice_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";
-
 // Short string.
 var s = "0123456789";
 // Long string.
 var l = "$s$s$s$s$s$s$s$s$s$s";
 // Very long string.
 var v = "$l$l$l$l$l$l$l$l$l$l";
 
 
 testSliceSuccess() {
   // Test different ways to get the same slice from a string.
@@ skipping 69 matching lines @@
   expectArgumentError(() => s.slice(-5, 4));
   expectArgumentError(() => s.slice(5, -6));
   expectArgumentError(() => s.slice(-5, -6));
 }
 
 
 main() {
   testSliceSuccess();
   testSliceError();
 }