Avoid runtime type errors in checked mode for ChunkedConverters.

sdk/lib/convert/base64.dart

Index: sdk/lib/convert/base64.dart
diff --git a/sdk/lib/convert/base64.dart b/sdk/lib/convert/base64.dart
index 3fe1357fb923058b5a948d7dbcff48819ba101a5..0254f853dbacd699704530b9be2596bbff3411b8 100644
--- a/sdk/lib/convert/base64.dart
+++ b/sdk/lib/convert/base64.dart
@@ -70,7 +70,9 @@ class Base64Codec extends Codec<List<int>, String> {
  *
  * 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;
@@ -340,7 +342,8 @@ class _Utf8Base64EncoderSink extends _Base64EncoderSink {
  *
  * 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();