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 56331e196eb3c13408054801885c67d4bb31e06f..09a35ce010d96f7286e5f00d36b9cec43c39f1d4 100644
--- a/sdk/lib/convert/base64.dart
+++ b/sdk/lib/convert/base64.dart
@@ -205,8 +205,7 @@ 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;
 
   const Base64Encoder() : _urlSafe = false;
@@ -465,8 +464,7 @@ 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();
 
   List<int> convert(String input, [int start = 0, int end]) {