Fix some strong mode issues in the core libraries.

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 cc586429012db4442df019391f62bde457c600ce..7eba8af9154251790913c15debde8065ecff24e7 100644
--- a/sdk/lib/convert/utf.dart
+++ b/sdk/lib/convert/utf.dart
@@ -76,8 +76,7 @@ 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>>
-    implements ChunkedConverter<String, List<int>, String, List<int>> {
+class Utf8Encoder extends Converter<String, List<int>> {
   const Utf8Encoder();
 
   /**
@@ -303,8 +302,7 @@ 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>
-    implements ChunkedConverter<List<int>, String, List<int>, String> {
+class Utf8Decoder extends Converter<List<int>, String> {
   final bool _allowMalformed;
 
   /**