Revert "Remove Expect from core library."

tests/standalone/crypto/base64_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";
-
 part "../../../sdk/lib/io/base64.dart";
 
 void main() {
   String line;
   String expected;
 
   line =
       "Man is distinguished, not only by his reason, but by this singular "
       "passion from other animals, which is a lust of the mind, that by a "
       "perseverance of delight in the continued and indefatigable generation "
@@ skipping 45 matching lines @@
   for (int i = 0; i < 251; i++) {
     List<int> x = [i, i + 1, i + 2, i + 3, i + 4, i + 5];
     Expect.listEquals(x, _Base64._decode(_Base64._encode(x)));
   }
 
   for (int i = 0; i < 250; i++) {
     List<int> x = [i, i + 1, i + 2, i + 3, i + 4, i + 5, i + 6];
     Expect.listEquals(x, _Base64._decode(_Base64._encode(x)));
   }
 }