Update deprecations.

lib/src/base64/decoder_sink.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 'decoder.dart';
 
-/// A [ChunkedConversionSink] for decoding chunks of Base64 strings to data.
+/// This is deprecated.
+///
+/// Use the `Base64Decoder` class in `dart:convert` instead.
+@Deprecated("Will be removed in crypto 1.0.0.")
 class Base64DecoderSink extends ChunkedConversionSink<String> {
   /// The encoder used to decode each chunk.
   final Base64Decoder _decoder = new Base64Decoder();
 
   /// The underlying sink to which to emit the decoded strings.
   final ChunkedConversionSink<List<int>> _outSink;
 
   /// The as-yet-unconverted text.
   ///
   /// This is used to handle text stopping partway through a four-character
@@ skipping 20 matching lines @@
     if (decodableLength > 0) {
       _outSink.add(_decoder.convert(chunk.substring(0, decodableLength)));
     }
   }
 
   void close() {
     if (_unconverted.isNotEmpty) _outSink.add(_decoder.convert(_unconverted));
     _outSink.close();
   }
 }