Avoid runtime type errors in checked mode for ChunkedConverters.

sdk/lib/convert/utf.dart

Index: sdk/lib/convert/utf.dart
diff --git a/sdk/lib/convert/utf.dart b/sdk/lib/convert/utf.dart
index 69bb72cb0f93ffac3b935a1523b6d1126a087ef1..aeee9ef8bf2806f71598341aaa695c2ceacb80a8 100644
--- a/sdk/lib/convert/utf.dart
+++ b/sdk/lib/convert/utf.dart
@@ -76,7 +76,8 @@ class Utf8Codec extends Encoding {
  * This class converts strings to their UTF-8 code units (a list of
  * unsigned 8-bit integers).
  */
-class Utf8Encoder extends Converter<String, List<int>> {
+class Utf8Encoder extends Converter<String, List<int>>
+    implements ChunkedConverter<String, List<int>, String, List<int>> {
 
   const Utf8Encoder();
 
@@ -304,7 +305,9 @@ class _Utf8EncoderSink extends _Utf8Encoder with StringConversionSinkMixin {
  * This class converts UTF-8 code units (lists of unsigned 8-bit integers)
  * to a string.
  */
-class Utf8Decoder extends Converter<List<int>, String> {
+class Utf8Decoder extends Converter<List<int>, String>
+    implements ChunkedConverter<List<int>, String, List<int>, String> {
+
   final bool _allowMalformed;
 
   /**