Fix some strong mode issues in the core libraries.

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 6634f9ef052e4db70733bc72a99adcb02d8646f7..ebbb44d09598840ae58d4c09ed23955a60c3f76c 100644
--- a/sdk/lib/convert/ascii.dart
+++ b/sdk/lib/convert/ascii.dart
@@ -69,8 +69,7 @@ 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>>
-    implements ChunkedConverter<String, List<int>, String, List<int>> {
+class _UnicodeSubsetEncoder extends Converter<String, List<int>> {
   final int _subsetMask;
 
   const _UnicodeSubsetEncoder(this._subsetMask);
@@ -155,8 +154,7 @@ 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>
-    implements ChunkedConverter<List<int>, String, List<int>, String> {
+abstract class _UnicodeSubsetDecoder extends Converter<List<int>, String> {
   final bool _allowInvalid;
   final int _subsetMask;