Make the package strong-mode clean.

lib/src/hex/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.
 
 library convert.hex.encoder;
 
 import 'dart:convert';
 import 'dart:typed_data';
 
 import 'package:charcode/ascii.dart';
 
 /// The canonical instance of [HexEncoder].
 const hexEncoder = const HexEncoder._();
 
 /// A converter that encodes byte arrays into hexadecimal strings.
 ///
 /// This will throw a [RangeError] if the byte array has any digits that don't
 /// fit in the gamut of a byte.
-class HexEncoder extends Converter<List<int>, String> {
+class HexEncoder
+    extends ChunkedConverter<List<int>, String, List<int>, String> {
   const HexEncoder._();
 
   String convert(List<int> bytes) => _convert(bytes, 0, bytes.length);
 
   ByteConversionSink startChunkedConversion(Sink<String> sink) =>
       new _HexEncoderSink(sink);
 }
 
 /// A conversion sink for chunked hexadecimal encoding.
 class _HexEncoderSink extends ByteConversionSinkBase {
@@ skipping 49 matching lines @@
         "Invalid byte ${byte < 0 ? "-" : ""}0x${byte.abs().toRadixString(16)}.",
         bytes, i);
   }
 
   throw 'unreachable';
 }
 
 /// Returns the ASCII/Unicode code unit corresponding to the hexadecimal digit
 /// [digit].
 int _codeUnitForDigit(int digit) => digit < 10 ? digit + $0 : digit + $a - 10;