Avoid runtime type errors in checked mode for ChunkedConverters.

sdk/lib/convert/base64.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.
 
 part of dart.convert;
 
 /**
  * A [base64](https://tools.ietf.org/html/rfc4648) encoder and decoder.
  *
  * It encodes using the default base64 alphabet,
@@ skipping 52 matching lines @@
 // Encoder
 // ------------------------------------------------------------------------
 
 /**
  * Base64 and base64url encoding converter.
  *
  * Encodes lists of bytes using base64 or base64url encoding.
  *
  * The results are ASCII strings using a restricted alphabet.
  */
-class Base64Encoder extends Converter<List<int>, String> {
+class Base64Encoder extends Converter<List<int>, String>
+    implements ChunkedConverter<List<int>, String, List<int>, String> {
+
   final bool _urlSafe;
 
   const Base64Encoder() : _urlSafe = false;
   const Base64Encoder.urlSafe() : _urlSafe = true;
 
   String convert(List<int> input) {
     if (input.isEmpty) return "";
     var encoder = new _Base64Encoder(_urlSafe);
     Uint8List buffer = encoder.encode(input, 0, input.length, true);
     return new String.fromCharCodes(buffer);
@@ skipping 249 matching lines @@
 // Decoder
 // ------------------------------------------------------------------------
 
 /**
  * Decoder for base64 encoded data.
  *
  * This decoder accepts both base64 and base64url ("url-safe") encodings.
  *
  * The encoding is required to be properly padded.
  */
-class Base64Decoder extends Converter<String, List<int>> {
+class Base64Decoder extends Converter<String, List<int>>
+    implements ChunkedConverter<String, List<int>, String, List<int>> {
 
   const Base64Decoder();
 
   List<int> convert(String input, [int start = 0, int end]) {
     end = RangeError.checkValidRange(start, end, input.length);
     if (start == end) return new Uint8List(0);
     var decoder = new _Base64Decoder();
     Uint8List buffer = decoder.decode(input, start, end);
     decoder.close(input, end);
     return buffer;
@@ skipping 387 matching lines @@
     end = RangeError.checkValidRange(start, end, string.length);
     if (start == end) return;
     Uint8List buffer = _decoder.decode(string, start, end);
     if (buffer != null) _sink.add(buffer);
     if (isLast) {
       _decoder.close(string, end);
       _sink.close();
     }
   }
 }