Make dart:convert strong mode clean.

sdk/lib/convert/base64.dart

Index: sdk/lib/convert/base64.dart
diff --git a/sdk/lib/convert/base64.dart b/sdk/lib/convert/base64.dart
index f3d7019af55b81f8bead51d2b55382aa34134228..3fe1357fb923058b5a948d7dbcff48819ba101a5 100644
--- a/sdk/lib/convert/base64.dart
+++ b/sdk/lib/convert/base64.dart
@@ -70,8 +70,7 @@ class Base64Codec extends Codec<List<int>, String> {
  *
  * The results are ASCII strings using a restricted alphabet.
  */
-class Base64Encoder extends
-    ChunkedConverter<List<int>, String, List<int>, String> {
+class Base64Encoder extends Converter<List<int>, String> {
   final bool _urlSafe;
 
   const Base64Encoder() : _urlSafe = false;
@@ -341,8 +340,7 @@ class _Utf8Base64EncoderSink extends _Base64EncoderSink {
  *
  * The encoding is required to be properly padded.
  */
-class Base64Decoder extends
-    ChunkedConverter<String, List<int>, String, List<int>> {
+class Base64Decoder extends Converter<String, List<int>> {
 
   const Base64Decoder();