Make convert library strong-mode compliant.

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 eb7519a2710175d9ace3aaaea344e44d9a498f8d..cd1f90a154f4125beebc5865d567bdea83c19b8b 100644
--- a/sdk/lib/convert/base64.dart
+++ b/sdk/lib/convert/base64.dart
@@ -53,7 +53,9 @@ class Base64Codec extends Codec<List<int>, String> {
  * Encodes lists of bytes using base64 encoding.
  * The result are ASCII strings using a restricted alphabet.
  */
-class Base64Encoder extends Converter<List<int>, String> {
+class Base64Encoder extends
+    ChunkedConverter<List<int>, String, List<int>, String> {
+
   const Base64Encoder();
 
   String convert(List<int> input) {
@@ -69,13 +71,6 @@ class Base64Encoder extends Converter<List<int>, String> {
     }
     return new _AsciiBase64EncoderSink(sink);
   }
-
-  Stream<String> bind(Stream<List<int>> stream) {
-    return new Stream<String>.eventTransformed(
-        stream,
-        (EventSink sink) =>
-            new _ConverterStreamEventSink<List<int>, String>(this, sink));
-  }
 }
 
 /**
@@ -304,7 +299,9 @@ class _Utf8Base64EncoderSink extends _Base64EncoderSink {
 // Decoder
 // ------------------------------------------------------------------------
 
-class Base64Decoder extends Converter<String, List<int>> {
+class Base64Decoder extends
+    ChunkedConverter<String, List<int>, String, List<int>> {
+
   const Base64Decoder();
 
   List<int> convert(String input, [int start = 0, int end]) {