Update deprecations.

lib/src/base64.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 'dart:convert';
 
 import 'base64/decoder.dart';
 import 'base64/encoder.dart';
 
-/// An instance of the default implementation of [Base64Codec].
+/// This is deprecated.
 ///
-/// This provides convenient access to most common Base64 use-cases.
+/// Use the `BASE64` constant in `dart:convert` instead.
+@Deprecated("Will be removed in crypto 1.0.0.")
 const Base64Codec BASE64 = const Base64Codec();
 
-/// A codec that converts between binary data and [Base64][rfc]-encoded strings.
+/// This is deprecated.
 ///
-/// [rfc]: https://tools.ietf.org/html/rfc4648
+/// Use the `Base64Codec` class in `dart:convert` instead.
+@Deprecated("Will be removed in crypto 1.0.0.")
 class Base64Codec extends Codec<List<int>, String> {
   final bool _urlSafe;
   final bool _addLineSeparator;
   final bool _encodePaddingCharacter;
 
   /// Creates a new [Base64Codec].
   ///
   /// The default [BASE64] codec will be good enough for most cases. A new codec
   /// only needs to be instantiated when you want to do multiple conversions
   /// with the same configuration.
@@ skipping 45 matching lines @@
         encodePaddingCharacter: encodePaddingCharacter).convert(bytes);
   }
 
   Base64Encoder get encoder => new Base64Encoder(
       urlSafe: _urlSafe,
       addLineSeparator: _addLineSeparator,
       encodePaddingCharacter: _encodePaddingCharacter);
 
   Base64Decoder get decoder => const Base64Decoder();
 }