Revert "Remove Expect from core library."

tests/lib/crypto/hmac_md5_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.
 
 // Library tag to allow the test to run on Dartium.
 library hmac_md5_test;
 
-import "package:expect/expect.dart";
 import 'dart:crypto';
 
 // Data from http://tools.ietf.org/html/rfc2202.
 var hmac_md5_inputs =
     const [
       const [ 0x48, 0x69, 0x20, 0x54, 0x68, 0x65, 0x72, 0x65 ],
       const [ 0x77, 0x68, 0x61, 0x74, 0x20, 0x64, 0x6f, 0x20, 0x79, 0x61,
               0x20, 0x77, 0x61, 0x6e, 0x74, 0x20, 0x66, 0x6f, 0x72, 0x20,
               0x6e, 0x6f, 0x74, 0x68, 0x69, 0x6e, 0x67, 0x3f ],
       const [ 0xdd, 0xdd, 0xdd, 0xdd, 0xdd, 0xdd, 0xdd, 0xdd, 0xdd, 0xdd,
@@ skipping 87 matching lines @@
     Expect.isTrue(h.verify(macs[i]));
     Expect.isFalse(h.verify(macs[(i+1)%macs.length]));
     Expect.throws(() => h.verify([]));
   }
 }
 
 void main() {
   testStandardVectors(hmac_md5_inputs, hmac_md5_keys,
                       hmac_md5_string_macs, hmac_md5_macs);
 }