Update deprecations.

lib/src/base64/encoder.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 'package:charcode/ascii.dart';
 
 import 'encoder_sink.dart';
 
 /// A String representing a mapping from numbers between 0 and 63, inclusive, to
 /// their corresponding encoded character.
 ///
 /// This is the table for URL-safe encodings.
 const _encodeTableUrlSafe =
     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
 
 /// A String representing a mapping from numbers between 0 and 63, inclusive, to
 /// their corresponding encoded character.
 ///
 /// This is the table for URL-unsafe encodings.
 const _encodeTable =
     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
 
 /// The line length for Base64 strings with line separators.
 const _lineLength = 76;
 
-/// An encoder that converts sequences of bytes to strings using [Base64][rfc].
+/// This is deprecated.
 ///
-/// [rfc]: https://tools.ietf.org/html/rfc4648
+/// Use the `Base64Encoder` class in `dart:convert` instead.
+@Deprecated("Will be removed in crypto 1.0.0.")
 class Base64Encoder extends Converter<List<int>, String> {
   /// Whether this encoder generates URL-safe strings.
   final bool _urlSafe;
 
   /// Whether this encoder adds line breaks to the output.
   final bool _addLineSeparator;
 
   /// The sequence of bytes to use as a padding character.
   final List<int> _pad;
 
@@ skipping 99 matching lines @@
   }
 
   Base64EncoderSink startChunkedConversion(Sink<String> sink) {
     StringConversionSink stringSink = sink is StringConversionSink
         ? sink
         : new StringConversionSink.from(sink);
 
     return new Base64EncoderSink(stringSink, _urlSafe, _addLineSeparator);
   }
 }