Fix some strong mode issues in the core libraries.

sdk/lib/io/string_transformer.dart

Index: sdk/lib/io/string_transformer.dart
diff --git a/sdk/lib/io/string_transformer.dart b/sdk/lib/io/string_transformer.dart
index 08b08bbb7c331baeb4c366573e36e8b0bd79fc44..fb864ad72024e33894d8267ecf1ef6e7bb4b4a6d 100644
--- a/sdk/lib/io/string_transformer.dart
+++ b/sdk/lib/io/string_transformer.dart
@@ -42,8 +42,7 @@ class SystemEncoding extends Encoding {
   }
 }
 
-class _WindowsCodePageEncoder extends Converter<String, List<int>>
-    implements ChunkedConverter<String, List<int>, String, List<int>> {
+class _WindowsCodePageEncoder extends Converter<String, List<int>> {
   const _WindowsCodePageEncoder();
 
   List<int> convert(String input) {
@@ -93,8 +92,7 @@ class _WindowsCodePageEncoderSink extends StringConversionSinkBase {
   }
 }
 
-class _WindowsCodePageDecoder extends Converter<List<int>, String>
-    implements ChunkedConverter<List<int>, String, List<int>, String> {
+class _WindowsCodePageDecoder extends Converter<List<int>, String> {
   const _WindowsCodePageDecoder();
 
   String convert(List<int> input) {