Fix some strong mode issues in the core libraries.

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 ee65ae7487ffd82d7ae994c3cffd834440642c00..e4212532be345428e1c0bb4436a0b2729d5503ca 100644
--- a/sdk/lib/convert/base64.dart
+++ b/sdk/lib/convert/base64.dart
@@ -204,8 +204,7 @@ class Base64Codec extends Codec<List<int>, String> {
  *
  * The results are ASCII strings using a restricted alphabet.
  */
-class Base64Encoder extends Converter<List<int>, String>
-    implements ChunkedConverter<List<int>, String, List<int>, String> {
+class Base64Encoder extends Converter<List<int>, String> {
 
   final bool _urlSafe;
 
@@ -476,8 +475,7 @@ class _Utf8Base64EncoderSink extends _Base64EncoderSink {
  *
  * The encoding is required to be properly padded.
  */
-class Base64Decoder extends Converter<String, List<int>>
-    implements ChunkedConverter<String, List<int>, String, List<int>> {
+class Base64Decoder extends Converter<String, List<int>> {
 
   const Base64Decoder();