Update deprecations.

lib/src/base64/decoder.dart

 // Copyright (c) 2015, 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 'dart:typed_data';
 
 import 'package:charcode/ascii.dart';
 
 import 'decoder_sink.dart';
 
 /// A mapping from ASCII character codes to their corresponding Base64 values.
 ///
 /// Characters with a value of `null` can't be decoded directly. This includes
 /// special values like CR, LF, `=`, and `%`.
 const _decodeTable = const [
   null, null, null, null, null, null, null, null, null, null, null, null, null,
   null, null, null, null, null, null, null, null, null, null, null, null, null,
   null, null, null, null, null, null, null, null, null, null, null, null, null,
   null, null, null, null, 62, null, 62, null, 63, 52, 53, 54, 55, 56, 57, 58,
   59, 60, 61, null, null, null, null, null, null, null, 0, 1, 2, 3, 4, 5, 6, 7,
   8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, null,
   null, null, null, 63, null, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37,
   38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51
 ];
 
-/// An encoder that converts [Base64][rfc] strings to sequences of bytes.
+/// This is deprecated.
 ///
-/// [rfc]: https://tools.ietf.org/html/rfc4648
+/// Use the `Base64Decoder` class in `dart:convert` instead.
+@Deprecated("Will be removed in crypto 1.0.0.")
 class Base64Decoder extends Converter<String, List<int>> {
   const Base64Decoder();
 
   List<int> convert(String input) {
     if (input.length == 0) return new Uint8List(0);
 
     // The length of the actual data sections in the input (not CRLFs).
     var dataLength = 0;
 
     // Count the data, and fail for invalid characters.
@@ skipping 78 matching lines @@
     }
 
     return out;
   }
 
   Base64DecoderSink startChunkedConversion(Sink<List<int>> sink) {
     if (sink is! ByteConversionSink) sink = new ByteConversionSink.from(sink);
     return new Base64DecoderSink(sink);
   }
 }