Avoid runtime type errors in checked mode for ChunkedConverters.

sdk/lib/convert/ascii.dart

Index: sdk/lib/convert/ascii.dart
diff --git a/sdk/lib/convert/ascii.dart b/sdk/lib/convert/ascii.dart
index a0762107cc19d028f27d5b2da16691a84050f63c..c954e643bab8c9bac377d9babff622637e4d8267 100644
--- a/sdk/lib/convert/ascii.dart
+++ b/sdk/lib/convert/ascii.dart
@@ -69,7 +69,9 @@ class AsciiCodec extends Encoding {
 
 // Superclass for [AsciiEncoder] and [Latin1Encoder].
 // Generalizes common operations that only differ by a mask;
-class _UnicodeSubsetEncoder extends Converter<String, List<int>> {
+class _UnicodeSubsetEncoder extends Converter<String, List<int>>
+    implements ChunkedConverter<String, List<int>, String, List<int>> {
+
   final int _subsetMask;
 
   const _UnicodeSubsetEncoder(this._subsetMask);
@@ -154,7 +156,9 @@ class _UnicodeSubsetEncoderSink extends StringConversionSinkBase {
  * This class converts Latin-1 bytes (lists of unsigned 8-bit integers)
  * to a string.
  */
-abstract class _UnicodeSubsetDecoder extends Converter<List<int>, String> {
+abstract class _UnicodeSubsetDecoder extends Converter<List<int>, String>
+    implements ChunkedConverter<List<int>, String, List<int>, String> {
+
   final bool _allowInvalid;
   final int _subsetMask;