Avoid runtime type errors in checked mode for ChunkedConverters.

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 ab17eafea121a94179b492ded02187ab2f26c3b3..1e540c35a234981114ab94f19113f3b0e289c041 100644
--- a/sdk/lib/io/string_transformer.dart
+++ b/sdk/lib/io/string_transformer.dart
@@ -42,7 +42,8 @@ class SystemEncoding extends Encoding {
   }
 }
 
-class _WindowsCodePageEncoder extends Converter<String, List<int>> {
+class _WindowsCodePageEncoder extends Converter<String, List<int>>
+    implements ChunkedConverter<String, List<int>, String, List<int>> {
 
   const _WindowsCodePageEncoder();
 
@@ -94,7 +95,8 @@ class _WindowsCodePageEncoderSink extends StringConversionSinkBase {
 }
 
 
-class _WindowsCodePageDecoder extends Converter<List<int>, String> {
+class _WindowsCodePageDecoder extends Converter<List<int>, String>
+    implements ChunkedConverter<List<int>, String, List<int>, String> {
 
   const _WindowsCodePageDecoder();